Record a trace event for method calls

Makes traces easier to navigate.

Currently only enabled when debug logging is on (ideally it should be
enabled when tracing is on, but OCaml doesn't tell us that).

CHANGES.md

@@ -44,7 +44,7 @@ New features:
 
 API changes:
 
-- Eio port (@talex5 #280 #284 #298 #292 #297 #300).
+- Eio port (@talex5 #280 #284 #298 #292 #297 #300 #304).
 
   This switches capnp-rpc from Lwt to Eio.
   One particularly nice side effect of this is that `Service.return_lwt` has gone,

capnp-rpc/service.ml

@@ -38,7 +38,10 @@ let local (s:#generic) =
       let call = Msg.Request.readable msg in
       let interface_id = Call.interface_id_get call in
       let method_id = Call.method_id_get call in
-      Log.debug (fun f -> f "Invoking local method %a" pp_method (interface_id, method_id));
+      Log.debug (fun f ->
+          Eio.Private.Trace.log (Fmt.str "%a" pp_method (interface_id, method_id));
+          f "Invoking local method %a" pp_method (interface_id, method_id)
+        );
       let p = Call.params_get call in
       let m : abstract_method_t = s#dispatch ~interface_id ~method_id in
       let release_params () = Core_types.Request_payload.release msg in