code: func (conn *BytesConnectionStr) SendAsyncString(topic string, key string, value string, meta interface{}, successClb func(*client.ProducerMessage), errClb func(*client.ProducerError)) {
  	conn.SendAsyncMessage(topic, sarama.StringEncoder(key), sarama.StringEncoder(value), meta, successClb, errClb)
  }
docs: // SendAsyncString sends a message that uses string encoder using the async API