prometheus: Add CollectorRegistry.unregister functions

Sources of metrics can sometimes disappear at runtime (a VM being
destroyed, disk being unplugged), and rather than continuing to present
the last value, it is useful to remove them altogether.

Other Prometheus client libraries do implement similar functions as
well, see https://docs.rs/prometheus/latest/prometheus/struct.Registry.html#method.unregister
for example.

Signed-off-by: Andrii Sultanov <andrii.sultanov@cloud.com>

Author: Andrii Sultanov

src/prometheus.ml

@@ -121,10 +121,16 @@ module CollectorRegistry = struct
     ensure_not_registered t info;
     t.metrics <- MetricFamilyMap.add info collector t.metrics
 
+  let unregister t info =
+    t.metrics <- MetricFamilyMap.remove info t.metrics
+
   let register_lwt t info collector =
     ensure_not_registered t info;
     t.metrics_lwt <- MetricFamilyMap.add info collector t.metrics_lwt
 
+  let unregister_lwt t info =
+    t.metrics_lwt <- MetricFamilyMap.remove info t.metrics_lwt
+
   open Lwt.Infix
 
   let map_p m =

src/prometheus.mli

@@ -89,10 +89,16 @@ module CollectorRegistry : sig
   (** [register t metric collector] adds [metric] to the set of metrics being collected.
       It will call [collector ()] to collect the values each time [collect] is called. *)
 
+  val unregister : t -> MetricInfo.t -> unit
+  (** [unregister t metric] removes [metric] from the set of metrics being collected. *)
+
   val register_lwt : t -> MetricInfo.t -> (unit -> Sample_set.t LabelSetMap.t Lwt.t) -> unit
   (** [register_lwt t metric collector] is the same as [register t metrics collector]
       but [collector] returns [Sample_set.t LabelSetMap.t Lwt.t]. *)
 
+  val unregister_lwt : t -> MetricInfo.t -> unit
+  (** [unregister_lwt t metric] removes [metric] from the set of metrics being collected. *)
+
   val register_pre_collect : t -> (unit -> unit) -> unit
   (** [register_pre_collect t fn] arranges for [fn ()] to be called at the start
       of each collection. This is useful if one expensive call provides