Assign partitions based on lag about kafkajs

[Nevon]

One thing I'm not sure about is how to calculate the offset lag per partition. Maybe I'm missing something, but I don't see a way to get this using ListOffset, etc.

[tulios]

You will have to use broker#offsetFetch because you want the group offset and not the topic offset.

[Nevon]

But that just gives you the group offset, but to calculate the lag you'd also need the highwater mark and the starting offset.

[tulios]

It's a combination of listOffset and offsetFetch

[Nevon]

I have a PoC implementation now that seems to work. I'll create a PR with it as soon as I've dealt with some error cases, and get some feedback. The assignment algorithm itself seems to produce balanced assignments. It's mostly calculating the offset lag that's not the nicest at the moment.

I'm not sure it makes sense to bundle it with the main distribution of KafkaJS though, as for most people the round-robin assigner would be fine. Since the assigner is pluggable anyway, I think it might make more sense to publish it as a separate module.