diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index f0bbcdf5..75829676 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -17,8 +17,9 @@ jobs:
             #- macos-latest
             #- windows-latest
         ocaml-compiler:
-          - 4.05
-          - 4.14
+          - 4.08.x
+          - 4.14.x
+          - 5.1.x
 
     runs-on: ${{ matrix.os }}
 
diff --git a/dune-project b/dune-project
index 415fe81c..435b7f75 100644
--- a/dune-project
+++ b/dune-project
@@ -13,11 +13,13 @@
   (name tiny_httpd)
   (synopsis "Minimal HTTP server using threads")
   (tags (http thread server tiny_httpd http_of_dir simplehttpserver))
+  (depopts
+    (mtime (>= 2.0)))
   (depends
     seq
     base-threads
     result
-    (ocaml (>= 4.05))
+    (ocaml (>= 4.08))
     (odoc :with-doc)
     (conf-libcurl :with-test)
     (ptime :with-test)
diff --git a/src/dune b/src/dune
index 89ba2327..680ebc43 100644
--- a/src/dune
+++ b/src/dune
@@ -7,7 +7,7 @@
 (library
   (name tiny_httpd)
   (public_name tiny_httpd)
-  (libraries threads seq)
+  (libraries threads seq unix)
   (wrapped false))
 
 (rule
diff --git a/src/prometheus/common_.ml b/src/prometheus/common_.ml
new file mode 100644
index 00000000..bb70b2d7
--- /dev/null
+++ b/src/prometheus/common_.ml
@@ -0,0 +1,3 @@
+module A = Tiny_httpd_atomic_
+
+let spf = Printf.sprintf
diff --git a/src/prometheus/dune b/src/prometheus/dune
new file mode 100644
index 00000000..5da724e5
--- /dev/null
+++ b/src/prometheus/dune
@@ -0,0 +1,12 @@
+
+
+(library
+ (name tiny_httpd_prometheus)
+ (public_name tiny_httpd.prometheus)
+ (synopsis "Metrics using prometheus")
+ (private_modules common_ time_)
+ (libraries
+  tiny_httpd unix
+  (select time_.ml from
+   (mtime mtime.clock.os -> time_.mtime.ml)
+   (-> time_.default.ml))))
diff --git a/src/prometheus/time_.default.ml b/src/prometheus/time_.default.ml
new file mode 100644
index 00000000..86dd302c
--- /dev/null
+++ b/src/prometheus/time_.default.ml
@@ -0,0 +1,3 @@
+let[@inline] now_us () =
+  let t = Unix.gettimeofday () in
+  t *. 1e6 |> ceil
diff --git a/src/prometheus/time_.mli b/src/prometheus/time_.mli
new file mode 100644
index 00000000..d6824fba
--- /dev/null
+++ b/src/prometheus/time_.mli
@@ -0,0 +1 @@
+val now_us : unit -> float
diff --git a/src/prometheus/time_.mtime.ml b/src/prometheus/time_.mtime.ml
new file mode 100644
index 00000000..65e2ec73
--- /dev/null
+++ b/src/prometheus/time_.mtime.ml
@@ -0,0 +1,3 @@
+let[@inline] now_us () =
+  let t = Mtime_clock.now_ns () in
+  Int64.(div t 1000L |> to_float)
diff --git a/src/prometheus/tiny_httpd_prometheus.ml b/src/prometheus/tiny_httpd_prometheus.ml
new file mode 100644
index 00000000..dcc19a8b
--- /dev/null
+++ b/src/prometheus/tiny_httpd_prometheus.ml
@@ -0,0 +1,240 @@
+(*
+  https://prometheus.io/docs/instrumenting/exposition_formats/#text-based-format
+  *)
+
+open Common_
+
+let bpf = Printf.bprintf
+
+type tags = (string * string) list
+type counter = { name: string; tags: tags; descr: string option; c: int A.t }
+type gauge = { name: string; tags: tags; descr: string option; g: int A.t }
+
+type histogram = {
+  name: string;
+  tags: tags;
+  descr: string option;
+  sum: float A.t;
+  buckets: (float * int A.t) array;
+}
+
+type registry = {
+  mutable counters: counter list;
+  mutable gauges: gauge list;
+  mutable hists: histogram list;
+  mutable on_will_emit: (unit -> unit) list;
+}
+
+let validate_descr_ what s =
+  if String.contains s '\n' then
+    invalid_arg (spf "%s: description cannot contain '\n'" what)
+
+let emit_tags_ buf tags =
+  if tags <> [] then (
+    bpf buf "{";
+    List.iteri
+      (fun i (k, v) ->
+        if i > 0 then bpf buf ",";
+        bpf buf "%s=%S" k v)
+      tags;
+    bpf buf "}"
+  )
+
+let opt_iter_ f = function
+  | None -> ()
+  | Some x -> f x
+
+module Counter = struct
+  type t = counter
+
+  let create (reg : registry) ?(tags = []) ?descr name : t =
+    let self : t = { name; descr; tags; c = A.make 0 } in
+    opt_iter_ (validate_descr_ "counter") descr;
+    reg.counters <- self :: reg.counters;
+    self
+
+  let emit buf (self : t) =
+    opt_iter_ (bpf buf "# HELP %s %s\n" self.name) self.descr;
+    bpf buf "# TYPE %s counter\n" self.name;
+    bpf buf "%s%a %d\n" self.name emit_tags_ self.tags (A.get self.c);
+    ()
+
+  let[@inline] incr self = A.incr self.c
+  let[@inline] incr_by self n = ignore (A.fetch_and_add self.c n : int)
+
+  let incr_to self n =
+    while
+      let old = A.get self.c in
+      if old < n then
+        not (A.compare_and_set self.c old n)
+      else
+        false
+    do
+      ()
+    done
+end
+
+module Gauge = struct
+  type t = gauge
+
+  let create (reg : registry) ?(tags = []) ?descr name : t =
+    opt_iter_ (validate_descr_ "gauge") descr;
+    let self : t = { name; descr; tags; g = A.make 0 } in
+    reg.gauges <- self :: reg.gauges;
+    self
+
+  let emit buf (self : t) =
+    opt_iter_ (bpf buf "# HELP %s %s\n" self.name) self.descr;
+    bpf buf "# TYPE %s gauge\n" self.name;
+    bpf buf "%s%a %d\n" self.name emit_tags_ self.tags (A.get self.g);
+    ()
+
+  let[@inline] set self x = A.set self.g x
+  let[@inline] incr self = A.incr self.g
+  let[@inline] incr_by self n = ignore (A.fetch_and_add self.g n : int)
+  let[@inline] decr self = A.decr self.g
+  let[@inline] decr_by self n = ignore (A.fetch_and_add self.g (-n) : int)
+end
+
+module Histogram = struct
+  type t = histogram
+
+  let create reg ?(tags = []) ?descr ~buckets name : t =
+    opt_iter_ (validate_descr_ "histogram") descr;
+    let buckets =
+      List.sort Stdlib.compare buckets
+      |> List.map (fun thresh -> thresh, A.make 0)
+    in
+    let buckets = Array.of_list @@ buckets @ [ infinity, A.make 0 ] in
+    let self : t = { name; descr; tags; sum = A.make 0.; buckets } in
+    reg.hists <- self :: reg.hists;
+    self
+
+  let add (self : t) n =
+    while
+      let old = A.get self.sum in
+      not (A.compare_and_set self.sum old (old +. n))
+    do
+      ()
+    done;
+    let i = ref 0 in
+    let continue = ref true in
+    while !continue && !i < Array.length self.buckets do
+      let thresh, count = self.buckets.(!i) in
+      if n <= thresh then (
+        continue := false;
+        A.incr count
+      ) else
+        incr i
+    done
+
+  let emit buf (self : t) : unit =
+    opt_iter_ (bpf buf "# HELP %s %s\n" self.name) self.descr;
+    bpf buf "# TYPE %s histogram\n" self.name;
+
+    let count = ref 0 in
+    for i = 0 to Array.length self.buckets - 1 do
+      let thresh, buck_count = self.buckets.(i) in
+      count := !count + A.get buck_count;
+
+      let name =
+        if thresh = infinity then
+          "+Inf"
+        else
+          string_of_float thresh
+      in
+      bpf buf "%s_bucket%a %d\n" self.name emit_tags_
+        (("le", name) :: self.tags)
+        !count
+    done;
+    bpf buf "%s_count%a %d\n" self.name emit_tags_ self.tags !count;
+    bpf buf "%s_sum%a %.3f\n" self.name emit_tags_ self.tags (A.get self.sum);
+    ()
+end
+
+module Registry = struct
+  type t = registry
+
+  let create () : t =
+    { counters = []; gauges = []; hists = []; on_will_emit = [] }
+
+  let on_will_emit self f = self.on_will_emit <- f :: self.on_will_emit
+
+  let emit (buf : Buffer.t) (self : t) : unit =
+    List.iter (fun f -> f ()) self.on_will_emit;
+    List.iter (Gauge.emit buf) self.gauges;
+    List.iter (Counter.emit buf) self.counters;
+    List.iter (Histogram.emit buf) self.hists;
+    ()
+
+  let emit_str (self : t) : string =
+    let buf = Buffer.create 32 in
+    emit buf self;
+    Buffer.contents buf
+end
+
+let global = Registry.create ()
+
+module H = Tiny_httpd
+
+let http_middleware (reg : Registry.t) : H.Middleware.t =
+  let c_req =
+    Counter.create reg "tiny_httpd_requests" ~descr:"number of HTTP requests"
+  in
+  let c_err =
+    Counter.create reg "tiny_httpd_errors" ~descr:"number of HTTP errors"
+  in
+  let h_latency =
+    Histogram.create reg "tiny_httpd_latency" ~descr:"latency of HTTP responses"
+      ~buckets:[ 0.001; 0.01; 0.1; 0.5; 1.; 5.; 10. ]
+  in
+
+  fun h : H.Middleware.handler ->
+    fun req ~resp : unit ->
+     let start = Time_.now_us () in
+     Counter.incr c_req;
+     h req ~resp:(fun (response : H.Response.t) ->
+         let code = response.code in
+
+         let elapsed_us = Time_.now_us () -. start in
+         let elapsed_s = elapsed_us /. 1e6 in
+         Histogram.add h_latency elapsed_s;
+
+         if code < 200 || code >= 300 then Counter.incr c_err;
+         resp response)
+
+let add_route_to_server (server : H.t) (reg : registry) : unit =
+  H.add_route_handler server H.Route.(exact "metrics" @/ return) @@ fun _req ->
+  let str = Registry.emit_str reg in
+  H.Response.make_string @@ Ok str
+
+let instrument_server (server : H.t) reg : unit =
+  H.add_middleware ~stage:(`Stage 1) server (http_middleware global);
+  add_route_to_server server reg
+
+module GC_metrics = struct
+  type t = { major_coll: counter; major_heap: gauge; compactions: counter }
+
+  let create reg : t =
+    let major_coll =
+      Counter.create reg ~descr:"major GC collections" "ocaml_gc_major"
+    in
+    let major_heap =
+      Gauge.create reg ~descr:"size of major heap" "ocaml_gc_major_heap_size"
+    in
+    let compactions =
+      Counter.create reg ~descr:"number of GC compactions"
+        "ocaml_gc_compactions"
+    in
+    { major_coll; major_heap; compactions }
+
+  let update (self : t) =
+    let stats = Gc.quick_stat () in
+    Counter.incr_to self.major_coll stats.major_collections;
+    Counter.incr_to self.compactions stats.compactions;
+    Gauge.set self.major_heap (stats.heap_words * 8)
+
+  let create_and_update_before_emit reg : unit =
+    let gc = create reg in
+    Registry.on_will_emit reg (fun () -> update gc)
+end
diff --git a/src/prometheus/tiny_httpd_prometheus.mli b/src/prometheus/tiny_httpd_prometheus.mli
new file mode 100644
index 00000000..799db5b2
--- /dev/null
+++ b/src/prometheus/tiny_httpd_prometheus.mli
@@ -0,0 +1,92 @@
+(** Expose metrics over HTTP in the prometheus format *)
+
+type tags = (string * string) list
+
+(** Registry for metrics. *)
+module Registry : sig
+  type t
+  (** The registry contains a group of metrics *)
+
+  val create : unit -> t
+
+  val on_will_emit : t -> (unit -> unit) -> unit
+  (** [on_will_emit registry f] calls [f()] every time
+      [emit buf registry] is called (before the metrics start being emitted). This
+      is useful to update some metrics on demand. *)
+
+  val emit : Buffer.t -> t -> unit
+  (** Write metrics into the given buffer. The buffer will be
+      cleared first thing. *)
+
+  val emit_str : t -> string
+end
+
+val global : Registry.t
+
+(** Counters *)
+module Counter : sig
+  type t
+  (** A counter, monotonically increasing *)
+
+  val create : Registry.t -> ?tags:tags -> ?descr:string -> string -> t
+  val incr : t -> unit
+  val incr_by : t -> int -> unit
+
+  val incr_to : t -> int -> unit
+  (** Increment to the given number. If it's lower than the current
+      value this does nothing *)
+end
+
+(** Gauges *)
+module Gauge : sig
+  type t
+  (** A gauge, taking arbitrary values *)
+
+  val create : Registry.t -> ?tags:tags -> ?descr:string -> string -> t
+  val set : t -> int -> unit
+  val incr : t -> unit
+  val incr_by : t -> int -> unit
+  val decr : t -> unit
+  val decr_by : t -> int -> unit
+end
+
+module Histogram : sig
+  type t
+  (** Histogram *)
+
+  val create :
+    Registry.t ->
+    ?tags:tags ->
+    ?descr:string ->
+    buckets:float list ->
+    string ->
+    t
+
+  val add : t -> float -> unit
+end
+
+(* TODO:
+   module Histogram : sig
+   end
+*)
+
+val http_middleware : Registry.t -> Tiny_httpd.Middleware.t
+(** Middleware to get basic metrics about HTTP requests *)
+
+val add_route_to_server : Tiny_httpd.t -> Registry.t -> unit
+(** Add a "/metrics" route to the server *)
+
+val instrument_server : Tiny_httpd.t -> Registry.t -> unit
+(** Add middleware and route *)
+
+module GC_metrics : sig
+  type t
+
+  val create : Registry.t -> t
+  val update : t -> unit
+
+  val create_and_update_before_emit : Registry.t -> unit
+  (** [create_and_update_before_emit reg] creates new GC metrics,
+    adds them to the registry, and uses {!Registry.on_will_emit}
+    to {!update} the metrics every time the registry is polled. *)
+end
diff --git a/tests/prometheus/dune b/tests/prometheus/dune
new file mode 100644
index 00000000..da921019
--- /dev/null
+++ b/tests/prometheus/dune
@@ -0,0 +1,4 @@
+
+(test
+  (name t_prom)
+  (libraries tiny_httpd.prometheus))
diff --git a/tests/prometheus/t_prom.expected b/tests/prometheus/t_prom.expected
new file mode 100644
index 00000000..1603900c
--- /dev/null
+++ b/tests/prometheus/t_prom.expected
@@ -0,0 +1,42 @@
+==== first try ====
+```
+# TYPE yolo_gauge gauge
+yolo_gauge{level="max"} 2525
+# HELP t_c2 more awesome than c1
+# TYPE t_c2 counter
+t_c2 1
+# TYPE t_c1 counter
+t_c1 42
+# HELP latency latency
+# TYPE latency histogram
+latency_bucket{le="0.01"} 2
+latency_bucket{le="0.1"} 4
+latency_bucket{le="0.5"} 7
+latency_bucket{le="1."} 8
+latency_bucket{le="10."} 9
+latency_bucket{le="+Inf"} 10
+latency_count 10
+latency_sum 31.530
+
+```
+==== second try====
+```
+# TYPE yolo_gauge gauge
+yolo_gauge{level="max"} 42000
+# HELP t_c2 more awesome than c1
+# TYPE t_c2 counter
+t_c2 2
+# TYPE t_c1 counter
+t_c1 53
+# HELP latency latency
+# TYPE latency histogram
+latency_bucket{le="0.01"} 2
+latency_bucket{le="0.1"} 4
+latency_bucket{le="0.5"} 8
+latency_bucket{le="1."} 9
+latency_bucket{le="10."} 10
+latency_bucket{le="+Inf"} 12
+latency_count 12
+latency_sum 54.930
+
+```
diff --git a/tests/prometheus/t_prom.ml b/tests/prometheus/t_prom.ml
new file mode 100644
index 00000000..04b1639e
--- /dev/null
+++ b/tests/prometheus/t_prom.ml
@@ -0,0 +1,40 @@
+module P = Tiny_httpd_prometheus
+
+let pf = Printf.printf
+let reg = P.Registry.create ()
+let c1 = P.Counter.create reg "t_c1"
+let c2 = P.Counter.create reg "t_c2" ~descr:"more awesome than c1"
+let g1 = P.Gauge.create reg ~tags:[ "level", "max" ] "yolo_gauge"
+
+let h1 =
+  P.Histogram.create reg ~descr:"latency"
+    ~buckets:[ 0.01; 0.1; 0.5; 1.; 10. ]
+    "latency"
+
+let () =
+  print_endline "==== first try ====";
+  P.Counter.incr_by c1 42;
+  P.Counter.incr c2;
+  P.Gauge.set g1 2525;
+  P.Histogram.add h1 0.2;
+  P.Histogram.add h1 0.003;
+  P.Histogram.add h1 0.002;
+  P.Histogram.add h1 0.025;
+  P.Histogram.add h1 0.9;
+  P.Histogram.add h1 7.4;
+  P.Histogram.add h1 22.2;
+  P.Histogram.add h1 0.3;
+  P.Histogram.add h1 0.4;
+  P.Histogram.add h1 0.1;
+
+  pf "```\n%s\n```\n" @@ P.Registry.emit_str reg
+
+let () =
+  print_endline "==== second try====";
+  P.Counter.incr_by c1 11;
+  P.Counter.incr c2;
+  P.Gauge.set g1 42_000;
+  P.Histogram.add h1 23.2;
+  P.Histogram.add h1 0.2;
+
+  pf "```\n%s\n```\n" @@ P.Registry.emit_str reg
diff --git a/tiny_httpd.opam b/tiny_httpd.opam
index 3cda25ee..955a14fe 100644
--- a/tiny_httpd.opam
+++ b/tiny_httpd.opam
@@ -15,12 +15,15 @@ depends: [
   "seq"
   "base-threads"
   "result"
-  "ocaml" {>= "4.05"}
+  "ocaml" {>= "4.08"}
   "odoc" {with-doc}
   "conf-libcurl" {with-test}
   "ptime" {with-test}
   "qcheck-core" {>= "0.9" & with-test}
 ]
+depopts: [
+  "mtime" {>= "2.0"}
+]
 build: [
   ["dune" "subst"] {dev}
   [