provide setServerSubscriberOption to brokers

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

context.go

@@ -4,6 +4,7 @@ import (
 	"context"
 
 	"github.com/micro/go-micro/broker"
+	"github.com/micro/go-micro/server"
 )
 
 // setSubscribeOption returns a function to setup a context with given value
@@ -26,6 +27,16 @@ func setBrokerOption(k, v interface{}) broker.Option {
 	}
 }
 
+// setBrokerOption returns a function to setup a context with given value
+func setServerSubscriberOption(k, v interface{}) server.SubscriberOption {
+	return func(o *server.SubscriberOptions) {
+		if o.Context == nil {
+			o.Context = context.Background()
+		}
+		o.Context = context.WithValue(o.Context, k, v)
+	}
+}
+
 // setPublishOption returns a function to setup a context with given value
 func setPublishOption(k, v interface{}) broker.PublishOption {
 	return func(o *broker.PublishOptions) {