[Backport 2.x] [Star tree] Star tree merge changes

[opensearch-trigger-bot]

Backport 312de99 from #14652.

[github-actions]

❌ Gradle check result for dc9d68f: 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 dc9d68f: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 78.20513% with 51 lines in your changes missing coverage. Please review.

Project coverage is 71.56%. Comparing base (768905f) to head (dc9d68f).
Report is 5 commits behind head on 2.x.

Files	Patch %	Lines
...ex/codec/composite/Composite99DocValuesWriter.java	68.88%	12 Missing and 2 partials ⚠️
...datacube/startree/builder/BaseStarTreeBuilder.java	81.25%	10 Missing and 2 partials ⚠️
...ex/datacube/startree/builder/StarTreesBuilder.java	52.38%	9 Missing and 1 partial ⚠️
...index/codec/composite/CompositeIndexFieldInfo.java	0.00%	6 Missing ⚠️
...be/startree/utils/SequentialDocValuesIterator.java	73.33%	2 Missing and 2 partials ⚠️
...acube/startree/aggregators/SumValueAggregator.java	66.66%	1 Missing and 1 partial ⚠️
...tacube/startree/builder/OnHeapStarTreeBuilder.java	96.49%	1 Missing and 1 partial ⚠️
...ec/composite/datacube/startree/StarTreeValues.java	90.90%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #15380      +/-   ##
============================================
- Coverage     71.64%   71.56%   -0.08%     
+ Complexity    63292    63267      -25     
============================================
  Files          5197     5197              
  Lines        296686   296808     +122     
  Branches      43177    43209      +32     
============================================
- Hits         212559   212410     -149     
- Misses        66307    66574     +267     
- Partials      17820    17824       +4     

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