code: public Future<RecordMetadata> sendMessageRaw(KafkaMessage message, Callback callback) {
          return sendMessageRaw(DEFAULT_PRODUCER_TYPE, message, callback);
      }
docs: Sends a message asynchronously, with default {@link ProducerType}.
  
  <p>
  This methods returns the underlying Kafka producer's output directly to
  caller, not converting {@link RecordMetadata} to {@link KafkaMessage}.
  </p>
  
  @param message
  @param callback
  @return
  @since 1.3.1