Metrics for shenandoah

[didip]

Based on this source code:

https://github.com/openjdk/jdk/blob/554caf33a01ac9ca2e3e9170557e8348750f3971/src/hotspot/share/gc/shenandoah/shenandoahMonitoringSupport.cpp#L65

Partially fixes #6424 by generating metrics for Shenandoah. Too bad ZGC does not have similar MonitoringSupport.cpp file.

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[xvrl]

@didip can you confirm which JVM versions this works with? Have you only tested on Java 11 or does this also work on Java 15?

[gianm]

@didip can you confirm which JVM versions this works with? Have you only tested on Java 11 or does this also work on Java 15?

I just checked and it looks like the names "Shenandoah partial" and "Shenandoah full" are used on both Java 11 and Java 15.

[didip]

@gianm yes, correct. Sorry for the late response, I am still on paternity leave.

[gianm]

@gianm yes, correct. Sorry for the late response, I am still on paternity leave.

No problem; and, congratulations 🙂

The only legit failure for this patch is test coverage. I'm planning on adding additional tests for JvmMonitor in #12469, so I'll merge this patch now and do all the new tests over there.