[Backport 2.x] [Tiered Caching] Additional ITs for cache stats

[peteralfonsi]

Original PR: #13655

Description

Adds more ITs and UT coverage for cache stats in the tiered spillover cache. Also has 3 small bugfixes which were found during testing:

• Cache clear API incorrectly wiped hits, misses, and eviction stats
• Items evicted from the heap tier, but rejected from the disk tier due to policies, incorrectly weren't counted towards the total evictions for the cache
• request_cache object in XContent response for the cache stats API was incorrectly at nodes.[node_id].request_cache instead of nodes.[node_id].caches.request_cache

Related Issues

Resolves #13455

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• API changes companion pull request created.
• 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 b3fc919: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 78.26087% with 5 lines in your changes missing coverage. Please review.

Project coverage is 71.28%. Comparing base (0dd892c) to head (3fc7716).
Report is 297 commits behind head on 2.x.

Files	Patch %	Lines
...pensearch/common/cache/service/NodeCacheStats.java	0.00%	3 Missing ⚠️
...e/common/tier/TieredSpilloverCacheStatsHolder.java	66.66%	1 Missing ⚠️
...va/org/opensearch/indices/IndicesRequestCache.java	90.90%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #13981      +/-   ##
============================================
- Coverage     71.28%   71.28%   -0.01%     
- Complexity    60145    61317    +1172     
============================================
  Files          4957     5039      +82     
  Lines        282799   288417    +5618     
  Branches      41409    42137     +728     
============================================
+ Hits         201591   205594    +4003     
- Misses        64189    65445    +1256     
- Partials      17019    17378     +359     

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[github-actions]

❌ Gradle check result for 51beda5: ABORTED

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 3fc7716: SUCCESS