code: func (ref *AsyncProducer) SendMsgToPartition(topic string, partition int32, key Encoder, msg Encoder, metadata interface{}) {
  	if msg == nil {
  		return
  	}
  
  	message := &sarama.ProducerMessage{
  		Topic:     topic,
  		Partition: partition,
  		Key:       key,
  		Value:     msg,
  		Metadata:  metadata,
  	}
  
  	ref.Producer.Input() <- message
  
  	ref.Debugf("message sent: %s", message)
  
  	return
  }
docs: // SendMsgToPartition sends an async message to Kafka