[Backport 2.x] [Tiered Caching] Stats rework (4/4): Adds stats implementation for TieredSpilloverCache

[opensearch-trigger-bot[bot]]

Backport a8017d84c45c5a32fbb35f84770ce4963cea8b1f from #13236.

[peteralfonsi]

@dblock @VachaShah @msfroh @andrross @reta Would any of you be able to approve this backport PR?

[github-actions[bot]]

:grey_exclamation: Gradle check result for 127db6eb63570015b29fcd5e10b6db2e65ee75e7: UNSTABLE

• TEST FAILURES:

      1 org.opensearch.cluster.coordination.AwarenessAttributeDecommissionIT.testConcurrentDecommissionAction

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