Changes

Summary

  1. Support export jvm metric (commit: cd6fc34) (details)
Commit cd6fc3448799bdbfb5c7980a4c62d2589962a7f2 by noreply
Support export jvm metric

Fix #13726

### What changes are proposed in this pull request?

Export JVM related metrics

### Why are the changes needed?

Add ClassLoadingGaugeSet and CachedThreadStatesGaugeSet to Alluxio
MetricRegistry.

### Does this PR introduce any user facing changes?

Add more metrics.

pr-link: Alluxio/alluxio#13762
change-id: cid-0bdad6a119292c20dc5378cfa8f2a104878cb46e
(commit: cd6fc34)
The file was modifiedcore/common/src/main/java/alluxio/metrics/MetricsSystem.java (diff)
The file was modifieddocs/en/reference/Metrics-List.md (diff)