KAFKA-9327: Document GroupMetadata metrics
KAFKA-9327: Document GroupMetadata metrics

This commit adds documentation on following `GroupMetadata` gauges:

- `NumOffsets`
- `NumGroups`
- `NumGroupsPreparingRebalance`
- `NumGroupsCompletingRebalance`
- `NumGroupsEmpty`
- `NumGroupsStable`
- `NumGroupsDead`

cc/ gwenshap

Author: Lee Dongjin <dongjin@apache.org>

Reviewers: Manikumar Reddy <manikumar.reddy@gmail.com>

Closes #7890 from dongjinleekr/feature/KAFKA-9327

apache/kafka,apache/kafka,TiVo/kafka,lindong28/kafka,Chasego/kafka,sslavic/kafka,apache/kafka,Chasego/kafka,guozhangwang/kafka,TiVo/kafka,guozhangwang/kafka,lindong28/kafka,lindong28/kafka,Chasego/kafka,apache/kafka,sslavic/kafka,Chasego/kafka,guozhangwang/kafka,lindong28/kafka,TiVo/kafka,sslavic/kafka,TiVo/kafka,sslavic/kafka,guozhangwang/kafka