code: func (conn *BytesConnectionStr) SendAsyncMessage(topic string, 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.hashAsyncProducer.SendMsgToPartition(topic, DefPartition, key, value, auxMeta)
  }
docs: // SendAsyncMessage sends a message using the async API and default partitioner