[Backport 2.x] [Bugfix] Fix TieredSpilloverCache stats not adding correctly when sha…

[peteralfonsi]

Backport c82cd2e from #16560.

Opened a manual backport as the earlier one was getting stuck and I couldn't fix merge conficts without write permissions. I've closed the other one.

[github-actions]

✅ Gradle check result for aeb226d: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.82%. Comparing base (81ee44a) to head (a45476a).
Report is 4 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...search/cache/common/tier/TieredSpilloverCache.java	75.00%	0 Missing and 1 partial ⚠️
...e/common/tier/TieredSpilloverCacheStatsHolder.java	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #16790      +/-   ##
============================================
- Coverage     72.01%   71.82%   -0.19%     
+ Complexity    65652    65568      -84     
============================================
  Files          5318     5318              
  Lines        305831   305836       +5     
  Branches      44601    44601              
============================================
- Hits         220233   219660     -573     
- Misses        67235    67814     +579     
+ Partials      18363    18362       -1     

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[jainankitk]

Thanks @peteralfonsi for raising this backport PR manually. Overall looks good, except for some of the seemingly unnecessary access modifiers. Can you fix that please, for main as well before merging this PR?

[peteralfonsi]

@jainankitk The access changes were made because the TSC stats holder and original stats holder are in different packages, and testing was a problem without them. But, I was able to rework the tests to use the ImmutableCacheStatsHolder to read any necessary values, so I've reverted the access modifiers to the way they were before this PR.

I'll raise the corresponding PR for main once we're happy with this version of the changes.

[github-actions]

✅ Gradle check result for 48e8c1a: SUCCESS

[opensearch-trigger-bot]

This PR is stalled because it has been open for 30 days with no activity.

[github-actions]

✅ Gradle check result for a45476a: 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:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-16790-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 eee491f8e907d6c6fcc70484fdcf39842ea61405
# Push it to GitHub
git push --set-upstream origin backport/backport-16790-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-16790-to-2.x.