feat: tracer for client-server debugging (#117)

Allow tracing of request/response to debug the interaction between client & server.

handler.go

@@ -141,8 +141,12 @@ type handler struct {
 	aliasedMethods map[string]string
 
 	paramDecoders map[reflect.Type]ParamDecoder
+
+	tracer Tracer
 }
 
+type Tracer func(method string, params []reflect.Value, results []reflect.Value, err error)
+
 func makeHandler(sc ServerConfig) *handler {
 	return &handler{
 		methods: make(map[string]methodHandler),
@@ -445,12 +449,18 @@ func (s *handler) handle(ctx context.Context, req request, w func(func(io.Writer
 	if err != nil {
 		rpcError(w, &req, 0, xerrors.Errorf("fatal error calling '%s': %w", req.Method, err))
 		stats.Record(ctx, metrics.RPCRequestError.M(1))
+		if s.tracer != nil {
+			s.tracer(req.Method, callParams, nil, err)
+		}
 		return
 	}
 	if req.ID == nil {
 		return // notification
 	}
 
+	if s.tracer != nil {
+		s.tracer(req.Method, callParams, callResult, nil)
+	}
 	// /////////////////
 
 	resp := response{

options_server.go

@@ -21,6 +21,7 @@ type ServerConfig struct {
 	errors        *Errors
 
 	reverseClientBuilder func(context.Context, *wsConn) (context.Context, error)
+	tracer               Tracer
 }
 
 type ServerOption func(c *ServerConfig)
@@ -58,6 +59,14 @@ func WithServerPingInterval(d time.Duration) ServerOption {
 	}
 }
 
+// WithTracer allows the instantiator to trace the method calls and results.
+// This is useful for debugging a client-server interaction.
+func WithTracer(l Tracer) ServerOption {
+	return func(c *ServerConfig) {
+		c.tracer = l
+	}
+}
+
 // WithReverseClient will allow extracting reverse client on **WEBSOCKET** calls.
 // RP is a proxy-struct type, much like the one passed to NewClient.
 func WithReverseClient[RP any](namespace string) ServerOption {