code: public Descriptor withMessageSerializer(Type messageType, MessageSerializer<?, ?> messageSerializer) {
          return replaceAllMessageSerializers(messageSerializers.plus(messageType, messageSerializer));
      }
docs: Provide a custom MessageSerializer for the given message type.
  
  @param messageType The type of the message.
  @param messageSerializer The message serializer for that type.
  @return A copy of this descriptor.