code: func NewHashProducer(p Producer, numPartitions int32) DistributingProducer {
  	return &hashProducer{
  		producer:   p,
  		partitions: numPartitions,
  	}
  }
docs: // NewHashProducer wraps given producer and return DistributingProducer that
  // publish messages to kafka, computing partition number from message key hash,
  // using fnv hash and [0, numPartitions) range.