Add pretty printer utils

These combinators seem tiny, but they simpflify code where they are used
quite a lot.

src/client/signal.ml

@@ -61,3 +61,13 @@ module Decode = struct
        ~dec:Trace_service.decode_pb_export_trace_service_request data)
       .resource_spans
 end
+
+module Pp = struct
+  module Proto = Opentelemetry.Proto
+
+  let logs = Format.pp_print_list Proto.Logs.pp_resource_logs
+
+  let metrics = Format.pp_print_list Proto.Metrics.pp_resource_metrics
+
+  let traces = Format.pp_print_list Proto.Trace.pp_resource_spans
+end

src/client/signal.mli

@@ -45,3 +45,16 @@ module Decode : sig
 
       @param encoder provide an encoder state to reuse *)
 end
+
+module Pp : sig
+  val logs :
+    Format.formatter -> Opentelemetry_proto.Logs.resource_logs list -> unit
+
+  val metrics :
+    Format.formatter ->
+    Opentelemetry_proto.Metrics.resource_metrics list ->
+    unit
+
+  val traces :
+    Format.formatter -> Opentelemetry_proto.Trace.resource_spans list -> unit
+end