[Backport 2.14] [Bugfix] Fix flaky test CacheStatsAPIIndicesRequestCacheIT.testNullLevels()

[peteralfonsi]

Original PR: #13457

2.x Backport PR: #13475

Description

I introduced CacheStatsAPIIndicesRequestCacheIT in this recent PR but I have found it to be flaky (example here). I created an issue for this here.

If levels is uninitialized in CommonStatsFlags, the String[] levels passed into DefaultCacheStatsHolder is usually an empty list. This is the expected behavior. But in some cases, DefaultCacheStatsHolder receives null instead of an empty list, and in this case, the XContent output derived from it has unexpected fields. To fix this we add an explicit null check when DefaultCacheStatsHolder constructs the object which produces XContent, and use an empty list if null is passed in.

We also initialize CommonStatsFlags.levels to be an empty String[] rather than null, as I found not doing this can rarely cause RestNodesStatsActionTests.testUnrecognizedMetric() to flake for the same reason.

Finally, fixes inconsistency with how null levels were used in tests.

Related Issues

Resolves #13458

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Failing checks are inspected and point to the corresponding known issue(s) (See: Troubleshooting Failing Builds)
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)
• Public documentation issue/PR created

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[github-actions]

❕ Gradle check result for af16613: UNSTABLE

• TEST FAILURES:

      1 org.opensearch.indices.replication.SegmentReplicationIT.testReplicaAlreadyAtCheckpoint
      1 org.opensearch.action.admin.indices.create.CreateIndexIT.testCreateAndDeleteIndexConcurrently
      1 org.opensearch.action.admin.indices.create.CreateIndexIT.classMethod

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (2.14@b8940cf). Click here to learn what that means.

Additional details and impacted files

@@           Coverage Diff           @@
##             2.14   #13485   +/-   ##
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  Branches        ?    42017           
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  Hits            ?   204675           
  Misses          ?    65607           
  Partials        ?    17191           

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