code: func sendMessage(producer *client.AsyncProducer, msg utils.Message) error {
  	var (
  		msgKey   []byte
  		msgMeta  []byte
  		msgValue []byte
  	)
  
  	// init message
  	if msg.Key != "" {
  		msgKey = []byte(msg.Key)
  	}
  	if msg.Metadata != "" {
  		msgMeta = []byte(msg.Metadata)
  	}
  	msgValue = []byte(msg.Text)
  
  	// send message
  	producer.SendMsgByte(msg.Topic, msgKey, msgValue, msgMeta)
  
  	fmt.Println("message sent")
  	return nil
  }
docs: // sendMessage demonstrates AsyncProducer.SendMsgByte API to publish a single
  // message to a Kafka topic.