[Remote Store] Fix stats api rejection count response

[ashking94]

Description

The backpressure_rejection_count field in the _remotestore/stats/{index} api response was coming to be zero always. The issue is narrowed to the missing code of incrementing the global rejection counter when the remote segment backpressure framework rejects a write request. In the PR, we have fixed the issue.

Related Issues

Resolves #7774

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/16359/
• CommitID: 45e58b1
  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?

[linuxpi]

Changes LGTM. Please take a look at the build failure

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/16360/
• CommitID: 8d3e76e
  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?

[ashking94]

Build failures would stop after #7739 is merged and this PR rebased with updated main.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.search.backpressure.SearchBackpressureIT.testSearchShardTaskCancellationWithHighCpu
      1 org.opensearch.search.SearchWeightedRoutingIT.testSearchAggregationWithNetworkDisruption_FailOpenEnabled
      1 org.opensearch.remotestore.RemoteStoreBackpressureIT.testWritesRejected

• URL: https://build.ci.opensearch.org/job/gradle-check/16363/
• CommitID: 29bbd40
  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

Merging #7775 (29bbd40) into main (adf7e2c) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main    #7775      +/-   ##
============================================
+ Coverage     70.72%   70.78%   +0.05%     
- Complexity    56146    56169      +23     
============================================
  Files          4682     4682              
  Lines        266102   266103       +1     
  Branches      39070    39070              
============================================
+ Hits         188193   188353     +160     
+ Misses        61935    61807     -128     
+ Partials      15974    15943      -31     

Impacted Files	Coverage Δ
...arch/index/remote/RemoteRefreshSegmentTracker.java	88.54% <100.00%> (+0.05%)	⬆️

... and 434 files with indirect coverage changes