code: func (conn *BytesManualConnectionStr) SendAsyncMessageToPartition(topic string, partition int32, key client.Encoder, value client.Encoder, meta interface{}, successClb func(*client.ProducerMessage), errClb func(*client.ProducerError)) {
  	auxMeta := &asyncMeta{successClb: successClb, errorClb: errClb, usersMeta: meta}
  	conn.multiplexer.manAsyncProducer.SendMsgToPartition(topic, partition, key, value, auxMeta)
  }
docs: // SendAsyncMessageToPartition sends a message using the async API and default partitioner