fix warnings

src/client/exporter_stdout.ml

@@ -1,7 +1,6 @@
 (** A simple exporter that prints on stdout *)
 
 open Common_
-open Opentelemetry_util
 open Opentelemetry_emitter
 
 open struct

src/core/span.mli

@@ -22,6 +22,8 @@ type kind = Span_kind.t =
 
 type key_value = Key_value.t
 
+val pp : Format.formatter -> t -> unit
+
 val make :
   ?kind:kind ->
   ?trace_state:string ->