Switch to ShardsBatchGatewayAllocator in batch enabled mode for shard…

[SwethaGuptha]

Description

Change in AllocationService to switch to ShardsBatchGatewayAllocator for serving API _cluster/allocation/explain when batch mode is enabled.

Related Issues

Resolves #[Issue number to be closed when this PR is merged]
#13990

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 27a1e00: FAILURE

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[github-actions]

❌ Gradle check result for 385dbe6: FAILURE

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[SwethaGuptha]

Failures:

[org.opensearch.indices.IndicesRequestCacheIT.testDynamicStalenessThresholdUpdate {p0={"opensearch.experimental.feature.pluggable.caching.enabled":"false"}}](https://build.ci.opensearch.org/job/gradle-check/40439/testReport/junit/org.opensearch.indices/IndicesRequestCacheIT/testDynamicStalenessThresholdUpdate__p0___opensearch_experimental_feature_pluggable_caching_enabled___false___/)
    [org.opensearch.indices.IndicesRequestCacheIT.classMethod](https://build.ci.opensearch.org/job/gradle-check/40439/testReport/junit/org.opensearch.indices/IndicesRequestCacheIT/classMethod/)

Flaky test:

#13949

[github-actions]

❌ Gradle check result for 385dbe6: FAILURE

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[github-actions]

❌ Gradle check result for 385dbe6: FAILURE

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[github-actions]

❌ Gradle check result for 385dbe6: FAILURE

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[SwethaGuptha]

Latest gradle run failure:

[Test Result](https://build.ci.opensearch.org/job/gradle-check/40647/testReport/) (1 failure / ±0)

    [org.opensearch.indices.IndicesRequestCacheIT.testStaleKeysRemovalWithoutExplicitThreshold {p0={"opensearch.experimental.feature.pluggable.caching.enabled":"false"}}](https://build.ci.opensearch.org/job/gradle-check/40647/testReport/junit/org.opensearch.indices/IndicesRequestCacheIT/testStaleKeysRemovalWithoutExplicitThreshold__p0___opensearch_experimental_feature_pluggable_caching_enabled___false___/)

flaky test: #14173

[github-actions]

❌ Gradle check result for 385dbe6: FAILURE

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[github-actions]

❌ Gradle check result for 385dbe6: FAILURE

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[github-actions]

❌ Gradle check result for 385dbe6: 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?

[SwethaGuptha]

❌ Gradle check result for 385dbe6: FAILURE

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#14173

[github-actions]

✅ Gradle check result for 385dbe6: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 71.71%. Comparing base (b15cb0c) to head (385dbe6).
Report is 404 commits behind head on main.

Files	Patch %	Lines
.../cluster/routing/allocation/AllocationService.java	25.00%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #14107      +/-   ##
============================================
+ Coverage     71.42%   71.71%   +0.29%     
- Complexity    59978    61429    +1451     
============================================
  Files          4985     5073      +88     
  Lines        282275   288527    +6252     
  Branches      40946    41777     +831     
============================================
+ Hits         201603   206923    +5320     
- Misses        63999    64464     +465     
- Partials      16673    17140     +467     

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