Document GroupMetadata metrics
This commit adds documentation on following `GroupMetadata` gauges:
`NumOffsets`
`NumGroups`
`NumGroupsPreparingRebalance`
`NumGroupsCompletingRebalance`
`NumGroupsEmpty`
`NumGroupsStable`
`NumGroupsDead`
cc/ gwenshap
Author: Lee Dongjin
Reviewers: Manikumar Reddy
Closes from dongjinleekr/feature/KAFKA-9327
<td>kafka.server:type=transaction-coordinator-metrics,name=partition-load-time-avg</td>
<td>average time, in milliseconds, it took to load transaction metadata from the consumer offset partitions loaded in the last 30 seconds</td>
</tr>
+ <tr>
+ <td>Consumer Group Offset Count</td>
+ <td>kafka.server:type=GroupMetadataManager,name=NumOffsets</td>
+ <td>Total number of committed offsets for Consumer Groups</td>
+ </tr>
+ <tr>
+ <td>Consumer Group Count</td>
+ <td>kafka.server:type=GroupMetadataManager,name=NumGroups</td>
+ <td>Total number of Consumer Groups</td>
+ </tr>
+ <tr>
+ <td>Consumer Group Count, per State</td>
+ <td>kafka.server:type=GroupMetadataManager,name=NumGroups[PreparingRebalance,CompletingRebalance,Empty,Stable,Dead]</td>
+ <td>The number of Consumer Groups in each state: PreparingRebalance, CompletingRebalance, Empty, Stable, Dead</td>
+ </tr>
</tbody></table>
<h4><a id="selector_monitoring" href="#selector_monitoring">Common monitoring metrics for producer/consumer/connect/streams</a></h4>
