refactor(cohttp): wrap in Server module

src/integrations/cohttp/README.md

@@ -1,11 +1,11 @@
 # Opentelemetry tracing for Cohttp_lwt servers
 
-Wrap your server callback with `Opentelemetry_cohttp_lwt.trace`:
+Wrap your server callback with `Opentelemetry_cohttp_lwt.Server.trace`:
 
 ```ocaml
 let my_server callback =
+  let callback =
+    Opentelemetry_cohttp_lwt.Server.trace ~service_name:"my-service" callback in
   Cohttp_lwt_unix.Server.create ~mode:(`TCP (`Port 8080))
-    (Server.make 
+    (Server.make () ~callback)
-      ~callback:(Opentelemetry_cohttp_lwt.trace ~service_name:"my-service" callback)
-      ())
 ```

src/integrations/cohttp/opentelemetry_cohttp_lwt.ml

@@ -3,13 +3,23 @@ module Otel_lwt = Opentelemetry_lwt
 open Cohttp
 open Cohttp_lwt
 
-type ('conn, 'body) callback =
+module Server : sig
-  'conn (* Cohttp_lwt_unix.Server.conn *)
+  (** Trace requests to a Cohttp server.
-  -> Request.t
-  -> 'body (* Cohttp_lwt.Body.t *)
-  -> (Response.t * 'body) Lwt.t
 
-let span_attrs (req : Request.t) =
+      Use it like this:
+
+          let my_server callback =
+            let callback =
+              Opentelemetry_cohttp_lwt.Server.trace ~service_name:"my-service" callback in
+            Cohttp_lwt_unix.Server.create ~mode:(`TCP (`Port 8080))
+              (Server.make () ~callback)
+   *)
+  val trace :
+    service_name:string ->
+    ('conn -> Request.t -> 'body -> (Response.t * 'body) Lwt.t) ->
+    'conn -> Request.t -> 'body -> (Response.t * 'body) Lwt.t
+end = struct
+  let span_attrs (req : Request.t) =
     let meth = req |> Request.meth |> Code.string_of_method in
     let referer = Header.get (Request.headers req) "referer" in
     let host = Header.get (Request.headers req) "host" in
@@ -31,7 +41,7 @@ let span_attrs (req : Request.t) =
              [ ("http.request.header.referer", `String r) ] )
       ]
 
-let trace_context_of_headers req =
+  let trace_context_of_headers req =
     let module Traceparent = Otel.Trace_context.Traceparent in
     match Header.get (Request.headers req) Traceparent.name with
     | None -> None, None
@@ -40,7 +50,7 @@ let trace_context_of_headers req =
         | Ok (trace_id, parent_id) -> (Some trace_id, Some parent_id)
         | Error _ -> None, None)
 
-let trace ~service_name (callback : ('conn, 'body) callback ) : ('conn, 'body) callback =
+  let trace ~service_name callback =
     fun conn req body ->
     let trace_id, parent_id = trace_context_of_headers req in
     let open Lwt.Syntax in
@@ -58,3 +68,4 @@ let trace ~service_name (callback : ('conn, 'body) callback ) : ('conn, 'body) c
             let code = Code.code_of_status code in
             [ ("http.status_code", `Int code) ]) ;
         Lwt.return (res, body) )
+end