Changing the signature of the partitioner

[mlowicki]

I would like to change

librdkafka/src/rdkafka_msg.c

Lines 1192 to 1205 in 966b63d

	if (!rkt->rkt_conf.random_partitioner &&
	(!rkm->rkm_key ||
	(rkm->rkm_key_len == 0 &&
	rkt->rkt_conf.partitioner ==
	rd_kafka_msg_partitioner_consistent_random))) {
	partition = rd_kafka_msg_sticky_partition(
	rkt, rkm->rkm_key, rkm->rkm_key_len,
	rkt->rkt_partition_cnt,
	rkt->rkt_conf.opaque, rkm->rkm_opaque);
	} else {
	partition = rkt->rkt_conf.partitioner(
	rkt, rkm->rkm_key, rkm->rkm_key_len,
	rkt->rkt_partition_cnt,
	rkt->rkt_conf.opaque, rkm->rkm_opaque);

to also pass payload size but it would violate https://github.com/mlowicki/librdkafka/blob/master/CONTRIBUTING.md#changes-to-existing-apis as e.g. signature of rd_kafka_topic_conf_set_partitioner_cb would change. Additional parameter could be used to implement more sophisticated partitioner like sticky partitioner based on the batch size so next partition will be elected when there is plenty data batched already to form big enough batch to sent to the current sticky partition.
Any thoughts on the change itself and how it could be rolled out to make sure we follow compatibility guarantees?