Add a SpanDecorator

go/otgrpc/client.go

@@ -63,6 +63,9 @@ func OpenTracingClientInterceptor(tracer opentracing.Tracer, optFuncs ...Option)
 			clientSpan.LogEventWithPayload("gRPC error", err)
 			ext.Error.Set(clientSpan, true)
 		}
+		if otgrpcOpts.decorator != nil {
+			otgrpcOpts.decorator(clientSpan, method, req, resp, err)
+		}
 		return err
 	}
 }

go/otgrpc/options.go

@@ -1,5 +1,7 @@
 package otgrpc
 
+import "github.com/opentracing/opentracing-go"
+
 // Option instances may be used in OpenTracing(Server|Client)Interceptor
 // initialization.
 //
@@ -15,11 +17,28 @@ func LogPayloads() Option {
 	}
 }
 
+// SpanDecoratorFunc provides an (optional) mechanism for otgrpc users to add
+// arbitrary tags/logs/etc to the opentracing.Span associated with client
+// and/or server RPCs.
+type SpanDecoratorFunc func(
+	span opentracing.Span,
+	method string,
+	req, resp interface{},
+	grpcError error)
+
+// SpanDecorator binds a function that decorates gRPC Spans.
+func SpanDecorator(decorator SpanDecoratorFunc) Option {
+	return func(o *options) {
+		o.decorator = decorator
+	}
+}
+
 // The internal-only options struct. Obviously overkill at the moment; but will
 // scale well as production use dictates other configuration and tuning
 // parameters.
 type options struct {
 	logPayloads bool
+	decorator   SpanDecoratorFunc
 }
 
 // newOptions returns the default options.

go/otgrpc/package.go

@@ -1,4 +1,4 @@
-// otgrpc provides OpenTracing support for any gRPC client or server in Go.
+// Package otgrpc provides OpenTracing support for any gRPC client or server.
 //
 // See [go/otgrpc/README.md](https://github.com/grpc-ecosystem/grpc-opentracing/go/otgrpc/README.md)
 package otgrpc

go/otgrpc/server.go

@@ -42,25 +42,28 @@ func OpenTracingServerInterceptor(tracer opentracing.Tracer, optFuncs ...Option)
 			// don't know where to put such an error and must rely on Tracer
 			// implementations to do something appropriate for the time being.
 		}
-		span := tracer.StartSpan(
+		serverSpan := tracer.StartSpan(
 			info.FullMethod,
 			ext.RPCServerOption(spanContext),
 			gRPCComponentTag,
 		)
-		defer span.Finish()
+		defer serverSpan.Finish()
 
-		ctx = opentracing.ContextWithSpan(ctx, span)
+		ctx = opentracing.ContextWithSpan(ctx, serverSpan)
 		if otgrpcOpts.logPayloads {
-			span.LogEventWithPayload("gRPC request", req)
+			serverSpan.LogEventWithPayload("gRPC request", req)
 		}
 		resp, err = handler(ctx, req)
 		if err == nil {
 			if otgrpcOpts.logPayloads {
-				span.LogEventWithPayload("gRPC response", resp)
+				serverSpan.LogEventWithPayload("gRPC response", resp)
 			}
 		} else {
-			ext.Error.Set(span, true)
-			span.LogEventWithPayload("gRPC error", err)
+			ext.Error.Set(serverSpan, true)
+			serverSpan.LogEventWithPayload("gRPC error", err)
+		}
+		if otgrpcOpts.decorator != nil {
+			otgrpcOpts.decorator(serverSpan, info.FullMethod, req, resp, err)
 		}
 		return resp, err
 	}