[Tiered Caching] Stats rework (3/4): Exposing new cache stats API

[peteralfonsi]

Description

This change exposes a new cache stats API described in this issue. Currently the only supported <cache_type> parameter is request_cache, but more can be added easily. This API supports stats aggregation by arbitrary dimensions, which are stored in the keys (ICacheKey objects) that are passed to ICache implementations. In the IRC, these dimensions are "indices", "shards", and "tier" (if a TieredSpilloverCache is used as the cache implementation, see #13236).

Tested manually and with an IT to check aggregation by various levels in the IRC.

No public doc PR for now, as the tiered caching feature is not yet complete and we are waiting to add doc PRs until it's fully ready for use.

Related Issues

Part of #12258

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 62d1c04: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❕ Gradle check result for 40796c5: UNSTABLE

• TEST FAILURES:

      1 org.opensearch.cluster.allocation.ClusterRerouteIT.testDelayWithALargeAmountOfShards

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

[github-actions]

❌ Gradle check result for 4ab4d7f: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for ecf780c: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for 3d1a1ee: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❕ Gradle check result for 5d76f0d: UNSTABLE

• TEST FAILURES:

      1 org.opensearch.remotestore.SegmentReplicationUsingRemoteStoreIT.testIndexReopenClose

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

[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-13237-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 2a37bddefe876a91ace2faf1a3cc91ebe0b7b6e5
# Push it to GitHub
git push --set-upstream origin backport/backport-13237-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-13237-to-2.x.