[Tiered Caching] Remove PLUGGABLE_CACHE feature flag

[peteralfonsi]

Description

As tiered caching will no longer be experimental in 3.0, removes its feature flag. Now users can always specify a store name via setting, and cache implementations will always track their own stats as exposed in GET _nodes/stats/caches/....

Modifies some UTs to account for this. Manually tested with debugger that the TSC can be used in the IRC via setting with the new changes. Also manually checked the response in GET _nodes/stats/caches/request_cache is populated even for the default cache.

Related Issues

Resolves #17343
Sub-issue of #17303

Check List

• Functionality includes testing.
• [N/A] API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for c5cf640: FAILURE

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❌ Gradle check result for 6126527: FAILURE

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[peteralfonsi]

Flaky test: #17291

[github-actions]

❌ Gradle check result for d212d39: FAILURE

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❌ Gradle check result for d95e24c: FAILURE

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❌ Gradle check result for 2a6794f: FAILURE

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[peteralfonsi]

Flaky tests: #17369

[github-actions]

❌ Gradle check result for 9c48b31: FAILURE

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[peteralfonsi]

Flaky tests: #14311, #17291

[github-actions]

❌ Gradle check result for 410f062: FAILURE

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[peteralfonsi]

Flaky tests: #17369, #14314, #15806

[github-actions]

❌ Gradle check result for 470251e: FAILURE

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[peteralfonsi]

Flaky test: #17291

[github-actions]

❕ Gradle check result for 8a559cb: UNSTABLE

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

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.47%. Comparing base (2d2b41d) to head (8a559cb).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
.../opensearch/common/cache/service/CacheService.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #17344      +/-   ##
============================================
+ Coverage     72.34%   72.47%   +0.13%     
- Complexity    65480    65598     +118     
============================================
  Files          5291     5291              
  Lines        304313   304313              
  Branches      44176    44174       -2     
============================================
+ Hits         220142   220566     +424     
+ Misses        66133    65690     -443     
- Partials      18038    18057      +19     

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