JMX QueueMetrics breaks after mutable config validation in CS

Reproduction steps:

1. Submit a long running job

hadoop-3.4.0-SNAPSHOT/bin/yarn jar hadoop-3.4.0-SNAPSHOT/share/hadoop/mapreduce/hadoop-mapreduce-client-jobclient-3.4.0-SNAPSHOT-tests.jar sleep -m 1 -r 1 -rt 1200000 -mt 20

2. Verify that there is one running app

$ curl http://localhost:8088/ws/v1/cluster/metrics | jq

3. Verify that the JMX endpoint reports 1 running app as well

$ curl http://localhost:8088/jmx | jq

4. Validate the configuration (x2)

$ curl -X POST -H 'Content-Type: application/json' -d @defaultqueue.json localhost:8088/ws/v1/cluster/scheduler-conf/validate

$ cat defaultqueue.json
{"update-queue":{"queue-name":"root.default","params":{"entry":{"key":"maximum-applications","value":"100"}}},"subClusterId":"","global":null,"global-updates":null}

5. Check 2. and 3. again. The cluster metrics should still work but the JMX endpoint will show 0 running apps, that's the bug.

It is caused by YARN-11211, reverting that patch (or only removing the QueueMetrics.clearQueueMetrics(); line) fixes the issue. But I think that would re-introduce the memory leak.

It looks like the QUEUE_METRICS hash map is "add-only", the clearQueueMetrics() was only called from ResourceManager.reinitialize() method (transitionToActive/transitionToStandby) prior to YARN-11211. Constantly adding and removing queues with unique names would cause a leak as well, because there is no remove from QUEUE_METRICS, so it is not just the validation API that has this problem.