diff --git a/dune-project b/dune-project
index 272aeb7c..a0833b62 100644
--- a/dune-project
+++ b/dune-project
@@ -55,3 +55,15 @@
   (odoc :with-doc)
   (cohttp-lwt (>= "4.0.0")))
  (synopsis "Opentelemetry tracing for Cohttp HTTP servers"))
+
+(package
+  (name opentelemetry-client-cohttp-lwt)
+  (depends
+   (ocaml (>= "4.08"))
+   (mtime (>= "1.4")) ; for spans
+   (opentelemetry (= :version))
+   (pbrt (>= 2.2))
+   (odoc :with-doc)
+   cohttp-lwt
+   cohttp-lwt-unix)
+  (synopsis "Collector client for opentelemetry, using cohttp + lwt"))
diff --git a/opentelemetry-client-cohttp-lwt.opam b/opentelemetry-client-cohttp-lwt.opam
new file mode 100644
index 00000000..70dae714
--- /dev/null
+++ b/opentelemetry-client-cohttp-lwt.opam
@@ -0,0 +1,36 @@
+# This file is generated by dune, edit dune-project instead
+opam-version: "2.0"
+version: "0.3"
+synopsis: "Collector client for opentelemetry, using cohttp + lwt"
+maintainer: ["the Imandra team and contributors"]
+authors: ["the Imandra team and contributors"]
+license: "MIT"
+homepage: "https://github.com/aestheticintegration/ocaml-opentelemetry"
+bug-reports:
+  "https://github.com/aestheticintegration/ocaml-opentelemetry/issues"
+depends: [
+  "dune" {>= "2.7"}
+  "ocaml" {>= "4.08"}
+  "mtime" {>= "1.4"}
+  "opentelemetry" {= version}
+  "pbrt" {>= "2.2"}
+  "odoc" {with-doc}
+  "cohttp-lwt"
+  "cohttp-lwt-unix"
+]
+build: [
+  ["dune" "subst"] {dev}
+  [
+    "dune"
+    "build"
+    "-p"
+    name
+    "-j"
+    jobs
+    "@install"
+    "@runtest" {with-test}
+    "@doc" {with-doc}
+  ]
+]
+dev-repo:
+  "git+https://github.com/aestheticintegration/ocaml-opentelemetry.git"
diff --git a/src/client-cohttp-lwt/common_.ml b/src/client-cohttp-lwt/common_.ml
new file mode 100644
index 00000000..4de037c4
--- /dev/null
+++ b/src/client-cohttp-lwt/common_.ml
@@ -0,0 +1,38 @@
+open Lwt.Syntax
+module Atomic = Opentelemetry_atomic.Atomic
+
+let[@inline] ( let@ ) f x = f x
+
+let spf = Printf.sprintf
+
+let tid () = Thread.id @@ Thread.self ()
+
+let debug_ =
+  ref
+    (match Sys.getenv_opt "OTEL_OCAML_DEBUG" with
+    | Some ("1" | "true") -> true
+    | _ -> false)
+
+let default_url = "http://localhost:4318"
+
+let url =
+  ref (try Sys.getenv "OTEL_EXPORTER_OTLP_ENDPOINT" with _ -> default_url)
+
+let get_url () = !url
+
+let set_url s = url := s
+
+let parse_headers s =
+  let parse_header s = Scanf.sscanf s "%s@=%s" (fun key value -> key, value) in
+  String.split_on_char ',' s |> List.map parse_header
+
+let default_headers = []
+
+let headers =
+  ref
+    (try parse_headers (Sys.getenv "OTEL_EXPORTER_OTLP_HEADERS")
+     with _ -> default_headers)
+
+let get_headers () = !headers
+
+let set_headers s = headers := s
diff --git a/src/client-cohttp-lwt/config.ml b/src/client-cohttp-lwt/config.ml
new file mode 100644
index 00000000..4e90283f
--- /dev/null
+++ b/src/client-cohttp-lwt/config.ml
@@ -0,0 +1,45 @@
+open Common_
+
+type t = {
+  debug: bool;
+  url: string;
+  headers: (string * string) list;
+  batch_traces: int option;
+  batch_metrics: int option;
+  batch_logs: int option;
+  batch_timeout_ms: int;
+}
+
+let pp out self : unit =
+  let ppiopt = Format.pp_print_option Format.pp_print_int in
+  let pp_header ppf (a, b) = Format.fprintf ppf "@[%s: @,%s@]@." a b in
+  let ppheaders = Format.pp_print_list pp_header in
+  let {
+    debug;
+    url;
+    headers;
+    batch_traces;
+    batch_metrics;
+    batch_logs;
+    batch_timeout_ms;
+  } =
+    self
+  in
+  Format.fprintf out
+    "{@[ debug=%B;@ url=%S;@ headers=%a;@ batch_traces=%a;@ batch_metrics=%a;@ \
+     batch_logs=%a;@ batch_timeout_ms=%d; @]}"
+    debug url ppheaders headers ppiopt batch_traces ppiopt batch_metrics ppiopt
+    batch_logs batch_timeout_ms
+
+let make ?(debug = !debug_) ?(url = get_url ()) ?(headers = get_headers ())
+    ?(batch_traces = Some 400) ?(batch_metrics = Some 20)
+    ?(batch_logs = Some 400) ?(batch_timeout_ms = 500) () : t =
+  {
+    debug;
+    url;
+    headers;
+    batch_traces;
+    batch_metrics;
+    batch_timeout_ms;
+    batch_logs;
+  }
diff --git a/src/client-cohttp-lwt/config.mli b/src/client-cohttp-lwt/config.mli
new file mode 100644
index 00000000..2aded994
--- /dev/null
+++ b/src/client-cohttp-lwt/config.mli
@@ -0,0 +1,54 @@
+type t = private {
+  debug: bool;
+  url: string;
+      (** Url of the endpoint. Default is "http://localhost:4318",
+      or "OTEL_EXPORTER_OTLP_ENDPOINT" if set. *)
+  headers: (string * string) list;
+      (** API headers sent to the endpoint. Default is none or
+      "OTEL_EXPORTER_OTLP_HEADERS" if set. *)
+  batch_traces: int option;
+      (** Batch traces? If [Some i], then this produces batches of (at most)
+      [i] items. If [None], there is no batching.
+
+      Note that traces and metrics are batched separately.
+      Default [Some 400].
+  *)
+  batch_metrics: int option;
+      (** Batch metrics? If [Some i], then this produces batches of (at most)
+      [i] items. If [None], there is no batching.
+
+      Note that traces and metrics are batched separately.
+      Default [None].
+  *)
+  batch_logs: int option;
+      (** Batch logs? See {!batch_metrics} for details.
+      Default [Some 400] *)
+  batch_timeout_ms: int;
+      (** Number of milliseconds after which we will emit a batch, even
+      incomplete.
+      Note that the batch might take longer than that, because this is
+      only checked when a new event occurs. Default 500. *)
+}
+(** Configuration.
+
+  To build one, use {!make} below. This might be extended with more
+  fields in the future. *)
+
+val make :
+  ?debug:bool ->
+  ?url:string ->
+  ?headers:(string * string) list ->
+  ?batch_traces:int option ->
+  ?batch_metrics:int option ->
+  ?batch_logs:int option ->
+  ?batch_timeout_ms:int ->
+  unit ->
+  t
+(** Make a configuration.
+
+   @param thread if true and [bg_threads] is not provided, we will pick a number
+   of bg threads. Otherwise the number of [bg_threads] superseeds this option.
+
+ *)
+
+val pp : Format.formatter -> t -> unit
diff --git a/src/client-cohttp-lwt/dune b/src/client-cohttp-lwt/dune
new file mode 100644
index 00000000..5faafe1a
--- /dev/null
+++ b/src/client-cohttp-lwt/dune
@@ -0,0 +1,6 @@
+(library
+ (name opentelemetry_client_cohttp_lwt)
+ (public_name opentelemetry-client-cohttp-lwt)
+ (synopsis "Opentelemetry collector using cohttp+lwt+unix")
+ (libraries opentelemetry lwt cohttp-lwt cohttp-lwt-unix pbrt mtime
+   mtime.clock.os))
diff --git a/src/client-cohttp-lwt/opentelemetry_client_cohttp_lwt.ml b/src/client-cohttp-lwt/opentelemetry_client_cohttp_lwt.ml
new file mode 100644
index 00000000..4e4afb86
--- /dev/null
+++ b/src/client-cohttp-lwt/opentelemetry_client_cohttp_lwt.ml
@@ -0,0 +1,578 @@
+(*
+   https://github.com/open-telemetry/oteps/blob/main/text/0035-opentelemetry-protocol.md
+   https://github.com/open-telemetry/oteps/blob/main/text/0099-otlp-http.md
+ *)
+
+module OT = Opentelemetry
+module Config = Config
+open Opentelemetry
+include Common_
+
+let needs_gc_metrics = Atomic.make false
+
+let gc_metrics = ref []
+(* side channel for GC, appended to {!E_metrics}'s data *)
+
+(* capture current GC metrics if {!needs_gc_metrics} is true,
+   and push them into {!gc_metrics} for later
+   collection *)
+let sample_gc_metrics_if_needed () =
+  if Atomic.compare_and_set needs_gc_metrics true false then (
+    let l =
+      OT.Metrics.make_resource_metrics
+        ~attrs:(Opentelemetry.GC_metrics.get_runtime_attributes ())
+      @@ Opentelemetry.GC_metrics.get_metrics ()
+    in
+    gc_metrics := l :: !gc_metrics
+  )
+
+type error =
+  [ `Status of int * Opentelemetry.Proto.Status.status
+  | `Failure of string
+  | `Sysbreak
+  ]
+
+let n_errors = Atomic.make 0
+
+let n_dropped = Atomic.make 0
+
+let report_err_ = function
+  | `Sysbreak -> Printf.eprintf "opentelemetry: ctrl-c captured, stopping\n%!"
+  | `Failure msg ->
+    Format.eprintf "@[<2>opentelemetry: export failed: %s@]@." msg
+  | `Status (code, { Opentelemetry.Proto.Status.code = scode; message; details })
+    ->
+    let pp_details out l =
+      List.iter
+        (fun s -> Format.fprintf out "%S;@ " (Bytes.unsafe_to_string s))
+        l
+    in
+    Format.eprintf
+      "@[<2>opentelemetry: export failed with@ http code=%d@ status \
+       {@[code=%ld;@ message=%S;@ details=[@[%a@]]@]}@]@."
+      code scode
+      (Bytes.unsafe_to_string message)
+      pp_details details
+
+module Httpc : sig
+  type t
+
+  val create : unit -> t
+
+  val send :
+    t ->
+    path:string ->
+    decode:[ `Dec of Pbrt.Decoder.t -> 'a | `Ret of 'a ] ->
+    string ->
+    ('a, error) result Lwt.t
+
+  val cleanup : t -> unit
+end = struct
+  open Opentelemetry.Proto
+  open Lwt.Syntax
+  module Httpc = Cohttp_lwt_unix.Client
+
+  type t = unit
+
+  let create () : t = ()
+
+  let cleanup _self = ()
+
+  (* send the content to the remote endpoint/path *)
+  let send (_self : t) ~path ~decode (bod : string) : ('a, error) result Lwt.t =
+    let full_url = !url ^ path in
+    let uri = Uri.of_string full_url in
+
+    let open Cohttp in
+    let headers = Header.(add_list (init ()) !headers) in
+    let headers =
+      Header.(add headers "Content-Type" "application/x-protobuf")
+    in
+
+    let body = Cohttp_lwt.Body.of_string bod in
+
+    let* r =
+      Lwt.catch
+        (fun () ->
+          let+ r = Httpc.post ~headers ~body uri in
+          Ok r)
+        (fun e -> Lwt.return @@ Error e)
+    in
+    match r with
+    | Error e ->
+      let err =
+        `Failure
+          (spf "sending signals via http POST to %S\nfailed with:\n%s" full_url
+             (Printexc.to_string e))
+      in
+      Lwt.return @@ Error err
+    | Ok (resp, body) ->
+      let* body = Cohttp_lwt.Body.to_string body in
+      let code = Response.status resp |> Code.code_of_status in
+      if not (Code.is_error code) then (
+        match decode with
+        | `Ret x -> Lwt.return @@ Ok x
+        | `Dec f ->
+          let dec = Pbrt.Decoder.of_string body in
+          let r =
+            try Ok (f dec)
+            with e ->
+              let bt = Printexc.get_backtrace () in
+              Error
+                (`Failure
+                  (spf "decoding failed with:\n%s\n%s" (Printexc.to_string e) bt))
+          in
+          Lwt.return r
+      ) else (
+        let dec = Pbrt.Decoder.of_string body in
+
+        let r =
+          try
+            let status = Status.decode_status dec in
+            Error (`Status (code, status))
+          with e ->
+            let bt = Printexc.get_backtrace () in
+            Error
+              (`Failure
+                (spf
+                   "httpc: decoding of status (url=%S, code=%d) failed with:\n\
+                    %s\n\
+                    status: %S\n\
+                    %s"
+                   full_url code (Printexc.to_string e) body bt))
+        in
+        Lwt.return r
+      )
+end
+
+(** Batch of resources to be pushed later.
+
+    This type is thread-safe. *)
+module Batch : sig
+  type 'a t
+
+  val push : 'a t -> 'a -> bool
+  (** [push batch x] pushes [x] into the batch, and heuristically
+      returns [true] if the batch is ready to be emitted (to know if we should
+      wake up the sending thread, if any) *)
+
+  val push' : 'a t -> 'a -> unit
+
+  val is_ready : now:Mtime.t -> _ t -> bool
+  (** is the batch ready to be sent? This is heuristic. *)
+
+  val pop_if_ready : ?force:bool -> now:Mtime.t -> 'a t -> 'a list option
+  (** Is the batch ready to be emitted? If batching is disabled,
+      this is true as soon as {!is_empty} is false. If a timeout is provided
+      for this batch, then it will be ready if an element has been in it
+      for at least the timeout.
+      @param now passed to implement timeout *)
+
+  val make : ?batch:int -> ?timeout:Mtime.span -> unit -> 'a t
+  (** Create a new batch *)
+end = struct
+  type 'a t = {
+    mutable size: int;
+    mutable q: 'a list;
+    batch: int option;
+    high_watermark: int;
+    timeout: Mtime.span option;
+    mutable start: Mtime.t;
+  }
+
+  let make ?batch ?timeout () : _ t =
+    Option.iter (fun b -> assert (b > 0)) batch;
+    let high_watermark = Option.fold ~none:100 ~some:(fun x -> x * 10) batch in
+    {
+      size = 0;
+      start = Mtime_clock.now ();
+      q = [];
+      batch;
+      timeout;
+      high_watermark;
+    }
+
+  let is_empty_ self = self.size = 0
+
+  let timeout_expired_ ~now self : bool =
+    match self.timeout with
+    | Some t ->
+      let elapsed = Mtime.span now self.start in
+      Mtime.Span.compare elapsed t >= 0
+    | None -> false
+
+  let is_full_ self : bool =
+    match self.batch with
+    | None -> self.size > 0
+    | Some b -> self.size >= b
+
+  let is_ready ~now self : bool = is_full_ self || timeout_expired_ ~now self
+
+  let pop_if_ready ?(force = false) ~now (self : _ t) : _ list option =
+    if self.size > 0 && (force || is_full_ self || timeout_expired_ ~now self)
+    then (
+      let l = self.q in
+      self.q <- [];
+      self.size <- 0;
+      assert (l <> []);
+      Some l
+    ) else
+      None
+
+  let push (self : _ t) x : bool =
+    if self.size >= self.high_watermark then (
+      (* drop this to prevent queue from growing too fast *)
+      Atomic.incr n_dropped;
+      true
+    ) else (
+      if self.size = 0 && Option.is_some self.timeout then
+        (* current batch starts now *)
+        self.start <- Mtime_clock.now ();
+
+      (* add to queue *)
+      self.size <- 1 + self.size;
+      self.q <- x :: self.q;
+      let ready = is_full_ self in
+      ready
+    )
+
+  let push' self x = ignore (push self x : bool)
+end
+
+(** An emitter. This is used by {!Backend} below to forward traces/metrics/…
+    from the program to whatever collector client we have. *)
+module type EMITTER = sig
+  open Opentelemetry.Proto
+
+  val push_trace : Trace.resource_spans list -> unit
+
+  val push_metrics : Metrics.resource_metrics list -> unit
+
+  val push_logs : Logs.resource_logs list -> unit
+
+  val set_on_tick_callbacks : (unit -> unit) list ref -> unit
+
+  val tick : unit -> unit
+
+  val cleanup : unit -> unit
+end
+
+(* make an emitter.
+
+   exceptions inside should be caught, see
+   https://opentelemetry.io/docs/reference/specification/error-handling/ *)
+let mk_emitter ~stop ~(config : Config.t) () : (module EMITTER) =
+  let open Proto in
+  let open Lwt.Syntax in
+  (* local helpers *)
+  let open struct
+    let timeout =
+      if config.batch_timeout_ms > 0 then
+        Some Mtime.Span.(config.batch_timeout_ms * ms)
+      else
+        None
+
+    let batch_traces : Trace.resource_spans list Batch.t =
+      Batch.make ?batch:config.batch_traces ?timeout ()
+
+    let batch_metrics : Metrics.resource_metrics list Batch.t =
+      Batch.make ?batch:config.batch_metrics ?timeout ()
+
+    let batch_logs : Logs.resource_logs list Batch.t =
+      Batch.make ?batch:config.batch_logs ?timeout ()
+
+    let on_tick_cbs_ = Atomic.make (ref [])
+
+    let set_on_tick_callbacks = Atomic.set on_tick_cbs_
+
+    let send_http_ (httpc : Httpc.t) encoder ~path ~encode x : unit Lwt.t =
+      Pbrt.Encoder.reset encoder;
+      encode x encoder;
+      let data = Pbrt.Encoder.to_string encoder in
+      let* r = Httpc.send httpc ~path ~decode:(`Ret ()) data in
+      match r with
+      | Ok () -> Lwt.return ()
+      | Error `Sysbreak ->
+        Printf.eprintf "ctrl-c captured, stopping\n%!";
+        Atomic.set stop true;
+        Lwt.return ()
+      | Error err ->
+        (* TODO: log error _via_ otel? *)
+        Atomic.incr n_errors;
+        report_err_ err;
+        (* avoid crazy error loop *)
+        Lwt_unix.sleep 3.
+
+    let send_metrics_http curl encoder (l : Metrics.resource_metrics list list)
+        =
+      let l = List.fold_left (fun acc l -> List.rev_append l acc) [] l in
+      let x =
+        Metrics_service.default_export_metrics_service_request
+          ~resource_metrics:l ()
+      in
+      send_http_ curl encoder ~path:"/v1/metrics"
+        ~encode:Metrics_service.encode_export_metrics_service_request x
+
+    let send_traces_http curl encoder (l : Trace.resource_spans list list) =
+      let l = List.fold_left (fun acc l -> List.rev_append l acc) [] l in
+      let x =
+        Trace_service.default_export_trace_service_request ~resource_spans:l ()
+      in
+      send_http_ curl encoder ~path:"/v1/traces"
+        ~encode:Trace_service.encode_export_trace_service_request x
+
+    let send_logs_http curl encoder (l : Logs.resource_logs list list) =
+      let l = List.fold_left (fun acc l -> List.rev_append l acc) [] l in
+      let x =
+        Logs_service.default_export_logs_service_request ~resource_logs:l ()
+      in
+      send_http_ curl encoder ~path:"/v1/logs"
+        ~encode:Logs_service.encode_export_logs_service_request x
+
+    (* emit metrics, if the batch is full or timeout lapsed *)
+    let emit_metrics_maybe ~now ?force httpc encoder : bool Lwt.t =
+      match Batch.pop_if_ready ?force ~now batch_metrics with
+      | None -> Lwt.return false
+      | Some l ->
+        let batch = !gc_metrics :: l in
+        gc_metrics := [];
+        let+ () = send_metrics_http httpc encoder batch in
+        true
+
+    let emit_traces_maybe ~now ?force httpc encoder : bool Lwt.t =
+      match Batch.pop_if_ready ?force ~now batch_traces with
+      | None -> Lwt.return false
+      | Some l ->
+        let+ () = send_traces_http httpc encoder l in
+        true
+
+    let emit_logs_maybe ~now ?force httpc encoder : bool Lwt.t =
+      match Batch.pop_if_ready ?force ~now batch_logs with
+      | None -> Lwt.return false
+      | Some l ->
+        let+ () = send_logs_http httpc encoder l in
+        true
+
+    let[@inline] guard_exn_ where f =
+      try f ()
+      with e ->
+        let bt = Printexc.get_backtrace () in
+        Printf.eprintf
+          "opentelemetry-curl: uncaught exception in %s: %s\n%s\n%!" where
+          (Printexc.to_string e) bt
+
+    let emit_all_force (httpc : Httpc.t) encoder : unit Lwt.t =
+      let now = Mtime_clock.now () in
+      let+ (_ : bool) = emit_traces_maybe ~now ~force:true httpc encoder
+      and+ (_ : bool) = emit_logs_maybe ~now ~force:true httpc encoder
+      and+ (_ : bool) = emit_metrics_maybe ~now ~force:true httpc encoder in
+      ()
+
+    let tick_common_ () =
+      if !debug_ then Printf.eprintf "tick (from %d)\n%!" (tid ());
+      sample_gc_metrics_if_needed ();
+      List.iter
+        (fun f ->
+          try f ()
+          with e ->
+            Printf.eprintf "on tick callback raised: %s\n"
+              (Printexc.to_string e))
+        !(Atomic.get on_tick_cbs_);
+      ()
+
+    (* thread that calls [tick()] regularly, to help enforce timeouts *)
+    let setup_ticker_thread ~tick ~finally () =
+      let rec tick_thread () =
+        if Atomic.get stop then (
+          finally ();
+          Lwt.return ()
+        ) else
+          let* () = Lwt_unix.sleep 0.5 in
+          let* () = tick () in
+          tick_thread ()
+      in
+      Lwt.async tick_thread
+  end in
+  let httpc = Httpc.create () in
+  let encoder = Pbrt.Encoder.create () in
+
+  let module M = struct
+    (* we make sure that this is thread-safe, even though we don't have a
+       background thread. There can still be a ticker thread, and there
+       can also be several user threads that produce spans and call
+       the emit functions. *)
+
+    let push_trace e =
+      let@ () = guard_exn_ "push trace" in
+      Batch.push' batch_traces e;
+      let now = Mtime_clock.now () in
+      Lwt.async (fun () ->
+          let+ (_ : bool) = emit_traces_maybe ~now httpc encoder in
+          ())
+
+    let push_metrics e =
+      let@ () = guard_exn_ "push metrics" in
+      sample_gc_metrics_if_needed ();
+      Batch.push' batch_metrics e;
+      let now = Mtime_clock.now () in
+      Lwt.async (fun () ->
+          let+ (_ : bool) = emit_metrics_maybe ~now httpc encoder in
+          ())
+
+    let push_logs e =
+      let@ () = guard_exn_ "push logs" in
+      Batch.push' batch_logs e;
+      let now = Mtime_clock.now () in
+      Lwt.async (fun () ->
+          let+ (_ : bool) = emit_logs_maybe ~now httpc encoder in
+          ())
+
+    let set_on_tick_callbacks = set_on_tick_callbacks
+
+    let tick_ () =
+      tick_common_ ();
+      sample_gc_metrics_if_needed ();
+      let now = Mtime_clock.now () in
+      let+ (_ : bool) = emit_traces_maybe ~now httpc encoder
+      and+ (_ : bool) = emit_logs_maybe ~now httpc encoder
+      and+ (_ : bool) = emit_metrics_maybe ~now httpc encoder in
+      ()
+
+    let () = setup_ticker_thread ~tick:tick_ ~finally:ignore ()
+
+    (* if called in a blocking context: work in the background *)
+    let tick () = Lwt.async tick_
+
+    let cleanup () =
+      if !debug_ then Printf.eprintf "opentelemetry: exiting…\n%!";
+      Lwt.async (fun () ->
+          let* () = emit_all_force httpc encoder in
+          Httpc.cleanup httpc;
+          Lwt.return ())
+  end in
+  (module M)
+
+module Backend (Arg : sig
+  val stop : bool Atomic.t
+
+  val config : Config.t
+end)
+() : Opentelemetry.Collector.BACKEND = struct
+  include (val mk_emitter ~stop:Arg.stop ~config:Arg.config ())
+
+  open Opentelemetry.Proto
+  open Opentelemetry.Collector
+
+  let send_trace : Trace.resource_spans list sender =
+    {
+      send =
+        (fun l ~ret ->
+          (if !debug_ then
+            let@ () = Lock.with_lock in
+            Format.eprintf "send spans %a@."
+              (Format.pp_print_list Trace.pp_resource_spans)
+              l);
+          push_trace l;
+          ret ());
+    }
+
+  let last_sent_metrics = Atomic.make (Mtime_clock.now ())
+
+  let timeout_sent_metrics = Mtime.Span.(5 * s)
+  (* send metrics from time to time *)
+
+  let signal_emit_gc_metrics () =
+    if !debug_ then
+      Printf.eprintf "opentelemetry: emit GC metrics requested\n%!";
+    Atomic.set needs_gc_metrics true
+
+  let additional_metrics () : Metrics.resource_metrics list =
+    (* add exporter metrics to the lot? *)
+    let last_emit = Atomic.get last_sent_metrics in
+    let now = Mtime_clock.now () in
+    let add_own_metrics =
+      let elapsed = Mtime.span last_emit now in
+      Mtime.Span.compare elapsed timeout_sent_metrics > 0
+    in
+
+    (* there is a possible race condition here, as several threads might update
+       metrics at the same time. But that's harmless. *)
+    if add_own_metrics then (
+      Atomic.set last_sent_metrics now;
+      let open OT.Metrics in
+      [
+        make_resource_metrics
+          [
+            sum ~name:"otel.export.dropped" ~is_monotonic:true
+              [
+                int
+                  ~start_time_unix_nano:(Mtime.to_uint64_ns last_emit)
+                  ~now:(Mtime.to_uint64_ns now) (Atomic.get n_dropped);
+              ];
+            sum ~name:"otel.export.errors" ~is_monotonic:true
+              [
+                int
+                  ~start_time_unix_nano:(Mtime.to_uint64_ns last_emit)
+                  ~now:(Mtime.to_uint64_ns now) (Atomic.get n_errors);
+              ];
+          ];
+      ]
+    ) else
+      []
+
+  let send_metrics : Metrics.resource_metrics list sender =
+    {
+      send =
+        (fun m ~ret ->
+          (if !debug_ then
+            let@ () = Lock.with_lock in
+            Format.eprintf "send metrics %a@."
+              (Format.pp_print_list Metrics.pp_resource_metrics)
+              m);
+
+          let m = List.rev_append (additional_metrics ()) m in
+          push_metrics m;
+          ret ());
+    }
+
+  let send_logs : Logs.resource_logs list sender =
+    {
+      send =
+        (fun m ~ret ->
+          (if !debug_ then
+            let@ () = Lock.with_lock in
+            Format.eprintf "send logs %a@."
+              (Format.pp_print_list Logs.pp_resource_logs)
+              m);
+
+          push_logs m;
+          ret ());
+    }
+end
+
+let setup_ ?(stop = Atomic.make false) ~(config : Config.t) () =
+  debug_ := config.debug;
+  let module B =
+    Backend
+      (struct
+        let stop = stop
+
+        let config = config
+      end)
+      ()
+  in
+  Opentelemetry.Collector.set_backend (module B);
+  B.cleanup
+
+let setup ?stop ?(config = Config.make ()) ?(enable = true) () =
+  if enable then (
+    let cleanup = setup_ ?stop ~config () in
+    at_exit cleanup
+  )
+
+let with_setup ?stop ?(config = Config.make ()) ?(enable = true) () f =
+  if enable then (
+    let cleanup = setup_ ?stop ~config () in
+    Fun.protect ~finally:cleanup f
+  ) else
+    f ()
diff --git a/src/client-cohttp-lwt/opentelemetry_client_cohttp_lwt.mli b/src/client-cohttp-lwt/opentelemetry_client_cohttp_lwt.mli
new file mode 100644
index 00000000..98536705
--- /dev/null
+++ b/src/client-cohttp-lwt/opentelemetry_client_cohttp_lwt.mli
@@ -0,0 +1,39 @@
+(*
+   TODO: more options from
+   https://opentelemetry.io/docs/reference/specification/protocol/exporter/
+   *)
+
+val get_url : unit -> string
+
+val set_url : string -> unit
+(** Url of the endpoint. Default is "http://localhost:4318",
+    or "OTEL_EXPORTER_OTLP_ENDPOINT" if set. *)
+
+val get_headers : unit -> (string * string) list
+
+val set_headers : (string * string) list -> unit
+(** Set http headers that are sent on every http query to the collector. *)
+
+module Config = Config
+
+val setup :
+  ?stop:bool Atomic.t -> ?config:Config.t -> ?enable:bool -> unit -> unit
+(** Setup endpoint. This modifies {!Opentelemetry.Collector.backend}.
+    @param enable actually setup the backend (default true). This can
+      be used to enable/disable the setup depending on CLI arguments
+      or environment.
+    @param config configuration to use
+    @param stop an atomic boolean. When it becomes true, background threads
+    will all stop after a little while.
+*)
+
+val with_setup :
+  ?stop:bool Atomic.t ->
+  ?config:Config.t ->
+  ?enable:bool ->
+  unit ->
+  (unit -> 'a) ->
+  'a
+(** [with_setup () f] is like [setup(); f()] but takes care of cleaning up
+    after [f()] returns
+    See {!setup} for more details. *)
diff --git a/src/opentelemetry.ml b/src/opentelemetry.ml
index f51d92e3..7378def4 100644
--- a/src/opentelemetry.ml
+++ b/src/opentelemetry.ml
@@ -981,8 +981,6 @@ end
 (** A set of callbacks that produce metrics when called.
     The metrics are automatically called regularly.
 
-
-
     This allows applications to register metrics callbacks from various points
     in the program (or even in libraries), and not worry about setting
     alarms/intervals to emit them. *)