While working on the new consumer and the coordinator, I found there are some consumer metadata info that should better be kept track of. For example, in the new consumer the consumer id is assigned by the coordinator, which is just the group name + index, and we have lost useful information such as host name in the consumer id. Adding a metadata in the consumer registry info as we did in the consumer commit message would be very useful in this case.
Since join group request protocol has not been exposed as the new consumer release, I think we can just change its format without bumping up request version. I am also wondering if a KIP is required for this change.