wip: opentelemetry.emitter with same time

a bit like a buffered writer for any data

src/emitter/dune

@@ -0,0 +1,5 @@
+(library
+ (name opentelemetry_emitter)
+ (public_name opentelemetry.emitter)
+ (libraries mtime mtime.clock.os)
+ (synopsis "Modular emitters for a single signal at a time"))

src/emitter/emitter.ml

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+(** Emitters *)
+
+exception Closed
+
+type 'a t = {
+  emit: 'a list -> unit;
+      (** Emit signals. @raise Closed if the emitter is closed. *)
+  tick: now:Mtime.t -> unit;
+      (** Call regularly to ensure background work is done *)
+  closed: unit -> bool;  (** True if the emitter was closed *)
+  flush_and_close: unit -> unit;
+      (** Flush internal buffered signals, then close *)
+}
+(** An emitter for values of type ['a]. *)
+
+let[@inline] emit (self : _ t) l : unit = if l <> [] then self.emit l
+
+let[@inline] tick (self : _ t) ~now : unit = self.tick ~now
+
+let[@inline] closed self : bool = self.closed ()
+
+let[@inline] flush_and_close (self : _ t) : unit = self.flush_and_close ()
+
+let map (f : 'a -> 'b) (self : 'b t) : 'a t =
+  { self with emit = (fun l -> self.emit (List.map f l)) }
+
+(* TODO: batching, either regular or sharded to reduce contention *)
+(* TODO: sampling *)
+
+(* TODO: use in Opentelemetry, and also for Tracer, Logger, etc. *)