add ?data to counter_int and counter_float

this makes sense to add metadata in, say, opentelemetry

src/core/collector.ml

@@ -71,10 +71,10 @@ module type S = sig
   val name_process : string -> unit
   (** Give a name to the current process. *)
 
-  val counter_int : string -> int -> unit
+  val counter_int : data:(string * user_data) list -> string -> int -> unit
   (** Integer counter. *)
 
-  val counter_float : string -> float -> unit
+  val counter_float : data:(string * user_data) list -> string -> float -> unit
   (** Float counter. *)
 
   val shutdown : unit -> unit

src/core/trace_core.ml

@@ -8,13 +8,15 @@ type collector = (module Collector.S)
 (** Global collector. *)
 let collector : collector option A.t = A.make None
 
+let data_empty_build_ () = []
+
 let[@inline] enabled () =
   match A.get collector with
   | None -> false
   | Some _ -> true
 
 let with_span_collector_ (module C : Collector.S) ?__FUNCTION__ ~__FILE__
-    ~__LINE__ ?(data = fun () -> []) name f =
+    ~__LINE__ ?(data = data_empty_build_) name f =
   let data = data () in
   C.with_span ~__FUNCTION__ ~__FILE__ ~__LINE__ ~data name f
 
@@ -28,7 +30,7 @@ let[@inline] with_span ?__FUNCTION__ ~__FILE__ ~__LINE__ ?data name f =
       f
 
 let enter_explicit_span_collector_ (module C : Collector.S) ~parent ~flavor
-    ?__FUNCTION__ ~__FILE__ ~__LINE__ ?(data = fun () -> []) name :
+    ?__FUNCTION__ ~__FILE__ ~__LINE__ ?(data = data_empty_build_) name :
     explicit_span =
   let data = data () in
   C.enter_manual_span ~parent ~flavor ~__FUNCTION__ ~__FILE__ ~__LINE__ ~data
@@ -69,8 +71,8 @@ let[@inline] add_data_to_manual_span esp data : unit =
     | Some (module C) -> C.add_data_to_manual_span esp data
   )
 
-let message_collector_ (module C : Collector.S) ?span ?(data = fun () -> []) msg
-    : unit =
+let message_collector_ (module C : Collector.S) ?span
+    ?(data = data_empty_build_) msg : unit =
   let data = data () in
   C.message ?span ~data msg
 
@@ -94,15 +96,19 @@ let messagef ?span ?data k =
             C.message ?span ~data str)
           fmt)
 
-let counter_int name n : unit =
+let counter_int ?(data = data_empty_build_) name n : unit =
   match A.get collector with
   | None -> ()
-  | Some (module C) -> C.counter_int name n
+  | Some (module C) ->
+    let data = data () in
+    C.counter_int ~data name n
 
-let counter_float name f : unit =
+let counter_float ?(data = data_empty_build_) name f : unit =
   match A.get collector with
   | None -> ()
-  | Some (module C) -> C.counter_float name f
+  | Some (module C) ->
+    let data = data () in
+    C.counter_float ~data name f
 
 let set_thread_name name : unit =
   match A.get collector with

src/core/trace_core.mli

@@ -110,12 +110,16 @@ val set_process_name : string -> unit
     This might be used by the collector
     to display traces in a more informative way. *)
 
-val counter_int : string -> int -> unit
+val counter_int :
+  ?data:(unit -> (string * user_data) list) -> string -> int -> unit
 (** Emit a counter of type [int]. Counters represent the evolution of some quantity
-    over time. *)
+    over time.
+    @param data metadata for this metric (since NEXT_RELEASE) *)
 
-val counter_float : string -> float -> unit
-(** Emit a counter of type [float]. See {!counter_int} for more details. *)
+val counter_float :
+  ?data:(unit -> (string * user_data) list) -> string -> float -> unit
+(** Emit a counter of type [float]. See {!counter_int} for more details.
+    @param data metadata for this metric (since NEXT_RELEASE) *)
 
 (** {2 Collector} *)
 

src/tef/trace_tef.ml

@@ -447,12 +447,12 @@ let collector ~out () : collector =
       let tid = get_tid_ () in
       B_queue.push events (E_message { tid; time_us; msg; data })
 
-    let counter_float name f =
+    let counter_float ~data:_ name f =
       let time_us = now_us () in
       let tid = get_tid_ () in
       B_queue.push events (E_counter { name; n = f; time_us; tid })
 
-    let counter_int name i = counter_float name (float_of_int i)
+    let counter_int ~data name i = counter_float ~data name (float_of_int i)
     let name_process name : unit = B_queue.push events (E_name_process { name })
 
     let name_thread name : unit =