Add committed offset metric per CG and partition

[tkornai]

Add a new committed offset metric per consumer group and per partition that can be used to judge if consumers are making progress on all partitions or if there is a poison pill on any of them.

Please let me know if exporting these metrics should be configurable.

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[weeco]

Hey @tkornai ,
thanks for your PR and sorry for the late review. I'm unsure if this new metric is valuable since we have a consumer group lag for each partition already. Even if you had this group offset metric you'd need to compare it with the partition offset to see if there are new messages that it should have committed to. And that's exactly what the consumer group lag metric already does. Maybe I'm missing something that cannot be solved using the existing metrics already?

My concern is that adding this metric could increase the partition cardinality quite a lot (depending on groups & partition count of course).