Making _cat/allocation API use indexLevelStats

[gargharsh3134]

Description

Backports #15292 to 2.17

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• Functionality includes testing.
• 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 75e470c: FAILURE

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 75e470c: 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 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 71.60%. Comparing base (0c3f2b4) to head (d9fbb10).
Report is 3 commits behind head on 2.17.

Files with missing lines	Patch %	Lines
...ensearch/rest/action/cat/RestAllocationAction.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               2.17   #15790      +/-   ##
============================================
- Coverage     71.61%   71.60%   -0.02%     
+ Complexity    64505    64466      -39     
============================================
  Files          5264     5264              
  Lines        301860   301861       +1     
  Branches      43942    43942              
============================================
- Hits         216178   216140      -38     
+ Misses        67637    67610      -27     
- Partials      18045    18111      +66     

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[github-actions]

❌ Gradle check result for d9fbb10: FAILURE

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 d9fbb10: SUCCESS