add consumer_member_id tag to partitions assign / revoked metrics

[mwarkentin]

Attempted fix for #357

DACI

Starting with the metrics most important for understanding consumer balancing across a group, but it may make sense to include this tag on other consumer-related metrics as well. Can be done in a follow up.

Questions

• [ ] Do we have consumer member ID available when we generate all metrics?
• [ ] Can we enable this "globally" in that case?
• [ ] If there are times when member ID is not available, could that just be recorded with a null or none tag?

[evanh]

Wouldn't this need to be applied to the rust version as well?

[mwarkentin]

Yes - my immediate concern was with a Python consumer so this is where I started.