code: func NewRoundRobinProducer(p Producer, numPartitions int32) DistributingProducer {
  	return &roundRobinProducer{
  		producer:   p,
  		partitions: numPartitions,
  		next:       0,
  	}
  }
docs: // NewRoundRobinProducer wraps given producer and return DistributingProducer
  // that publish messages to kafka, choosing destination partition from cycle
  // build from [0, numPartitions) range.