fix: emit GC metrics even in the absence of custom metrics

src/client-ocurl/opentelemetry_client_ocurl.ml

@@ -348,7 +348,9 @@ end = struct
         Queue.clear local_q;
 
         if !must_flush_all then (
-          if Batch.len batches.metrics > 0 then send_metrics ();
+          if Batch.len batches.metrics > 0 || not (AList.is_empty gc_metrics)
+          then
+            send_metrics ();
           if Batch.len batches.logs > 0 then send_logs ();
           if Batch.len batches.traces > 0 then send_traces ()
         ) else (

src/core/AList.ml

@@ -4,6 +4,11 @@ type 'a t = 'a list Atomic.t
 
 let make () = Atomic.make []
 
+let[@inline] is_empty self : bool =
+  match Atomic.get self with
+  | [] -> true
+  | _ :: _ -> false
+
 let get = Atomic.get
 
 let add self x =

src/core/AList.mli

@@ -5,6 +5,8 @@ type 'a t
 val get : 'a t -> 'a list
 (** Snapshot *)
 
+val is_empty : _ t -> bool
+
 val make : unit -> 'a t
 
 val add : 'a t -> 'a -> unit