Allow optional reuse of encoder state

This allows clien implementors to micromanage the state of the protobuf
`encoder` if they want, or to just let the library handle allocation and
garbage collection of encoder states when that level of resource
managment is not required.

src/client-cohttp-lwt/opentelemetry_client_cohttp_lwt.ml

@@ -271,7 +271,7 @@ end
 let mk_emitter ~stop ~(config : Config.t) () : (module EMITTER) =
   let open Proto in
   let open Lwt.Syntax in
-  let module Conv = Signal.Converter () in
+  let module Conv = Signal.Converter in
   (* local helpers *)
   let open struct
     let timeout =

src/client-ocurl/opentelemetry_client_ocurl.ml

@@ -201,7 +201,7 @@ end = struct
       in
       conv l |> send_http_ ~stop ~config ~url client
     in
-    let module Conv = Signal.Converter () in
+    let module Conv = Signal.Converter in
     try
       while not (Atomic.get stop) do
         let msg = B_queue.pop self.send_q in

src/client/signal.ml

@@ -5,34 +5,37 @@ module Span = Opentelemetry.Span
 
 let ( let@ ) = ( @@ )
 
-module Converter () = struct
-  let encoder = Pbrt.Encoder.create ()
-
-  let resource_to_string ~ctor ~enc resource =
+module Converter = struct
+  let resource_to_string ~encoder ~ctor ~enc resource =
+    let encoder =
+      match encoder with
+      | Some e -> e
+      | None -> Pbrt.Encoder.create ()
+    in
     let x = ctor resource in
     let@ _sc = Self_trace.with_ ~kind:Span.Span_kind_internal "encode-proto" in
     Pbrt.Encoder.reset encoder;
     enc x encoder;
     Pbrt.Encoder.to_string encoder
 
-  let logs resource_logs =
+  let logs ?encoder resource_logs =
     resource_logs
-    |> resource_to_string
+    |> resource_to_string ~encoder
          ~ctor:(fun r ->
            Logs_service.default_export_logs_service_request ~resource_logs:r ())
          ~enc:Logs_service.encode_pb_export_logs_service_request
 
-  let metrics resource_metrics =
+  let metrics ?encoder resource_metrics =
     resource_metrics
-    |> resource_to_string
+    |> resource_to_string ~encoder
          ~ctor:(fun r ->
            Metrics_service.default_export_metrics_service_request
              ~resource_metrics:r ())
          ~enc:Metrics_service.encode_pb_export_metrics_service_request
 
-  let traces resource_spans =
+  let traces ?encoder resource_spans =
     resource_spans
-    |> resource_to_string
+    |> resource_to_string ~encoder
          ~ctor:(fun r ->
            Trace_service.default_export_trace_service_request ~resource_spans:r
              ())

src/client/signal.mli

@@ -5,13 +5,27 @@
 
     NOTE: The converters share an underlying stateful encoder, so each domain or
     system thread should have its own [Converter] instance *)
-module Converter : functor () -> sig
-  val logs : Opentelemetry_proto.Logs.resource_logs list -> string
-  (** [logs ls] is a protobuf encoded string of the logs [ls] *)
+module Converter : sig
+  val logs :
+    ?encoder:Pbrt.Encoder.t ->
+    Opentelemetry_proto.Logs.resource_logs list ->
+    string
+  (** [logs ls] is a protobuf encoded string of the logs [ls]
 
-  val metrics : Opentelemetry_proto.Metrics.resource_metrics list -> string
-  (** [metrics ms] is a protobuf encoded string of the metrics [ms] *)
+      @param encoder provide an encoder state to reuse *)
 
-  val traces : Opentelemetry_proto.Trace.resource_spans list -> string
-  (** [metrics ts] is a protobuf encoded string of the traces [ts] *)
+  val metrics :
+    ?encoder:Pbrt.Encoder.t ->
+    Opentelemetry_proto.Metrics.resource_metrics list ->
+    string
+  (** [metrics ms] is a protobuf encoded string of the metrics [ms]
+      @param encoder provide an encoder state to reuse *)
+
+  val traces :
+    ?encoder:Pbrt.Encoder.t ->
+    Opentelemetry_proto.Trace.resource_spans list ->
+    string
+  (** [metrics ts] is a protobuf encoded string of the traces [ts]
+
+      @param encoder provide an encoder state to reuse *)
 end