Router: publish messages in bulk (#513)

Currently, the router will publish produced messaged by calling Publish individually, even though the bulk API exists.

This change works the same, although it can be handy for some custom implementations when you want to treat the produced messages as a group. For some implementations, it could also slightly improve publish performance.

message/router.go

@@ -825,15 +825,12 @@ func (h *handler) publishProducedMessages(producedMessages Messages, msgFields w
 		"publish_topic":           h.publishTopic,
 	}))
 
-	for _, msg := range producedMessages {
-		if err := h.publisher.Publish(h.publishTopic, msg); err != nil {
-			// todo - how to deal with it better/transactional/retry?
-			h.logger.Error("Cannot publish message", err, msgFields.Add(watermill.LogFields{
-				"not_sent_message": fmt.Sprintf("%#v", producedMessages),
-			}))
-
-			return err
-		}
+	if err := h.publisher.Publish(h.publishTopic, producedMessages...); err != nil {
+		h.logger.Error("Cannot publish messages", err, msgFields.Add(watermill.LogFields{
+			"not_sent_message": fmt.Sprintf("%#v", producedMessages),
+		}))
+
+		return err
 	}
 
 	return nil