[Tiered Caching] Stats rework (2/4): Gates CacheStatsHolder logic behind FeatureFlags.PLUGGABLE_CACHE setting

[peteralfonsi]

Description

Part of tiered caching stats. Tiered/pluggable caching is currently an experimental feature with the feature flag PLUGGABLE_CACHE. We want to gate the new CacheStatsHolder logic, which is run in the search path within cache implementations, behind this feature flag in case of any unexpected behavior.

With this PR, if PLUGGABLE_CACHE is true, cache implementations use the existing CacheStatsHolder class to keep track of their stats. If it's false, they instead use a DummyCacheStatsHolder, which keeps track of count only. In this case, the new cache stats API exposed in this PR will return all-zero stats and not support aggregating by any levels.

Because TieredSpilloverCache will store its own stats in #13236, NoopCacheStatsHolder will also be used in the individual tiers of the TieredSpilloverCache to avoid storing redundant stats.

Related Issues

Follow up to #12531.

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)
  - [N/A] Public documentation issue/PR created

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[github-actions]

❌ Gradle check result for bd0c2e5: FAILURE

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[github-actions]

❌ Gradle check result for a4b860d: FAILURE

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[github-actions]

❌ Gradle check result for 8d2914e: FAILURE

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[github-actions]

❌ Gradle check result for 3164464: FAILURE

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[github-actions]

✅ Gradle check result for c1d7bf9: SUCCESS

[jainankitk]

LGTM!

[github-actions]

❌ Gradle check result for 2abb89d: FAILURE

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[github-actions]

✅ Gradle check result for c708bc7: SUCCESS

[github-actions]

❌ Gradle check result for 97374fe: FAILURE

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[github-actions]

❌ Gradle check result for 239b711: FAILURE

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[github-actions]

❌ Gradle check result for 87da3ef: FAILURE

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[github-actions]

❌ Gradle check result for 9b0935f: FAILURE

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[github-actions]

✅ Gradle check result for f935dde: SUCCESS

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch/backport-2.x
# Create a new branch
git switch --create backport/backport-13238-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 f84d28d4371dbd00356cccc01366d87d21814f36
# Push it to GitHub
git push --set-upstream origin backport/backport-13238-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-13238-to-2.x.