use an interval limiter for the metrics_callbacks

src/lib/interval_limiter.mli

@@ -1,3 +1,6 @@
+(** Interval limiter. This is a form of rate limiting where an event cannot be
+    followed by another event until a given interval of time has passed. *)
+
 type t
 
 val create : min_interval:Mtime.span -> unit -> t

src/lib/metrics_callbacks.ml

@@ -6,8 +6,9 @@ let create () : t = { cbs = Alist.make () }
 
 let[@inline] add_metrics_cb (self : t) f = Alist.add self.cbs f
 
-let add_to_exporter (exp : Exporter.t) (self : t) =
+let minimum_min_interval = Mtime.Span.(100 * ms)
-  let on_tick () =
+
+let collect_and_send (self : t) (exp : Exporter.t) =
   (* collect all metrics *)
   let res = ref [] in
   List.iter
@@ -19,18 +20,33 @@ let add_to_exporter (exp : Exporter.t) (self : t) =
 
   (* emit the metrics *)
   Exporter.send_metrics exp metrics
+
+let add_to_exporter ?(min_interval = Mtime.Span.(4 * s)) (exp : Exporter.t)
+    (self : t) =
+  let min_interval =
+    Mtime.Span.(
+      if is_shorter min_interval ~than:minimum_min_interval then
+        minimum_min_interval
+      else
+        min_interval)
+  in
+
+  let limiter = Interval_limiter.create ~min_interval () in
+  let on_tick () =
+    if Interval_limiter.make_attempt limiter then collect_and_send self exp
   in
   Exporter.on_tick exp on_tick
 
-let with_set_added_to_exporter (exp : Exporter.t) (f : t -> 'a) : 'a =
+let with_set_added_to_exporter ?min_interval (exp : Exporter.t) (f : t -> 'a) :
+    'a =
   let set = create () in
-  add_to_exporter exp set;
+  add_to_exporter ?min_interval exp set;
   f set
 
-let with_set_added_to_main_exporter (f : t -> unit) : unit =
+let with_set_added_to_main_exporter ?min_interval (f : t -> unit) : unit =
   match Main_exporter.get () with
   | None -> ()
-  | Some exp -> with_set_added_to_exporter exp f
+  | Some exp -> with_set_added_to_exporter ?min_interval exp f
 
 module Main_set = struct
   let cur_set_ : t option Atomic.t = Atomic.make None

src/lib/metrics_callbacks.mli

@@ -16,14 +16,20 @@ val add_metrics_cb : t -> (unit -> Metrics.t list) -> unit
     metrics. It might be called regularly by the backend, in particular (but not
     only) when {!Exporter.tick} is called. *)
 
-val add_to_exporter : Exporter.t -> t -> unit
+val add_to_exporter : ?min_interval:Mtime.span -> Exporter.t -> t -> unit
-(** Make sure we export metrics at every [tick] of the exporter *)
+(** Make sure we try to export metrics at every [tick] of the exporter.
+    @param min_interval
+      the minimum duration between two consecutive exports, using
+      {!Interval_limiter}. We don't want a too frequent [tick] to spam metrics.
+      Default [4s], minimum [0.1s]. *)
 
-val with_set_added_to_exporter : Exporter.t -> (t -> 'a) -> 'a
+val with_set_added_to_exporter :
+  ?min_interval:Mtime.span -> Exporter.t -> (t -> 'a) -> 'a
 (** [with_set_added_to_exporter exp f] creates a set, adds it to the exporter,
     and calls [f] on it *)
 
-val with_set_added_to_main_exporter : (t -> unit) -> unit
+val with_set_added_to_main_exporter :
+  ?min_interval:Mtime.span -> (t -> unit) -> unit
 (** If there is a main exporter, add a set to it and call [f set], else do not
     call [f] at all *)