Rebalance algorithm should consider partitions from all topics

The current rebalance step, as stated in the original Kafka paper [1], splits the partitions per topic between all the consumers. So if you have 100 topics with 2 partitions each and 10 consumers only two consumers will be used. That is, for each topic all partitions will be listed and shared between the consumers in the consumer group in order (not randomly).

If the consumer group is reading from several topics at the same time it makes sense to split all the partitions from all topics between all the consumer. Following the example, we will have 200 partitions in total, 20 per consumer, using the 10 consumers.

The load per topic could be different and the division should consider this. However even a random division should be better than the current algorithm while reading from several topics and should harm reading from a few topics with several partitions.