Optimize NodeIndicesStats output behind flag

[Pranshu-S]

Description:

As of today the APIs which fetch Node Stats with indices stats currently returns all shard statistics regardless of the requested level (node, level, or indices) to the coordinator node (node where-in or from which the rest request was created). This unnecessary iteration on the coordinator node leads to overhead and potential health issues. Furthermore, the coordinator node repeats index-level statistics calculation for each node response even when the requested level is indices.

This pull request proposes optimizations to the NodeStats API to pre-compute and return only the necessary information to generate the response. The APIs would now perform pre-computation of shard or index-level statistics on remote nodes (individual nodes which respond to the coordinator node) based on the level parameter in the REST request - nodes, indices or shards. Only for level = shards does the response contains the shard level stats.

The optimisation hides behind a flag transport level flag optimizeNodeIndicesStatsOnLevel part of CommonStatsFlag. This is enabled in RestNodeStatsAction as well as RestNodeAction API calls as we do not require it during these code paths.

Pre-computation on remote nodes minimizes data transfer and processing on the coordinator node, leading to significant performance gains, especially in large clusters. Reduced load on the coordinator node contributes to a healthier state by minimizing GC pauses and CPU spikes.

Testing:

Extensive testing with a 20k shards cluster has validated the effectiveness of these optimizations.

Stats	Without Optimisation	With Optimisation
Average	0.857280048	0.4961762
P90	1.1514673	0.7631599
Max	1.78542	1.423496
Min	0.444184	0.212655

Related Issues

Resolves #13340

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[Pranshu-S]

[Test Result](https://build.ci.opensearch.org/job/gradle-check/42419/testReport/) (3 failures / -5)
[org.opensearch.discovery.ClusterDisruptionIT.classMethod](https://build.ci.opensearch.org/job/gradle-check/42419/testReport/junit/org.opensearch.discovery/ClusterDisruptionIT/classMethod/)
[org.opensearch.index.ShardIndexingPressureSettingsIT.testShardIndexingPressureLastSuccessfulSettingsUpdate](https://build.ci.opensearch.org/job/gradle-check/42419/testReport/junit/org.opensearch.index/ShardIndexingPressureSettingsIT/testShardIndexingPressureLastSuccessfulSettingsUpdate/)
[org.opensearch.indices.replication.SegmentReplicationIT.testReplicaAlreadyAtCheckpoint](https://build.ci.opensearch.org/job/gradle-check/42419/testReport/junit/org.opensearch.indices.replication/SegmentReplicationIT/testReplicaAlreadyAtCheckpoint/)

For org.opensearch.discovery.ClusterDisruptionIT.classMethod

java.lang.AssertionError: SpanData validation failed for validator org.opensearch.test.telemetry.tracing.validators.AllSpansAreEndedProperly
MockSpanData{spanID='aa1534630900e459', parentSpanID='f8c436a5e84c915f', traceID='d7fd33dd4c374128', spanName='dispatchedShardOperationOnPrimary', startEpochNanos=2792950755631, endEpochNanos=0, hasEnded=false, attributes={index=restart_while_indexing, refresh_policy=false, thread.name=opensearch[node_t1][write][T#1], bulk_request_items=1, shard_id=0, node_id=uRqO2n91TTCp5ocLhE5MQA}, stackTrace=null}

Similar to failures in Flaky Test Issue - #14308

---

For org.opensearch.index.ShardIndexingPressureSettingsIT.testShardIndexingPressureLastSuccessfulSettingsUpdate

java.lang.AssertionError: expected:<0> but was:<2>
	at org.junit.Assert.fail(Assert.java:89)
	at org.junit.Assert.failNotEquals(Assert.java:835)
	at org.junit.Assert.assertEquals(Assert.java:647)
	at org.junit.Assert.assertEquals(Assert.java:633)
	at org.opensearch.index.ShardIndexingPressureSettingsIT.lambda$waitForTwoOutstandingRequests$13(ShardIndexingPressureSettingsIT.java:939)
	at org.opensearch.test.OpenSearchTestCase.assertBusy(OpenSearchTestCase.java:1128)
	at org.opensearch.test.OpenSearchTestCase.assertBusy(OpenSearchTestCase.java:1101)
	at org.opensearch.index.ShardIndexingPressureSettingsIT.waitForTwoOutstandingRequests(ShardIndexingPressureSettingsIT.java:938)
	at 

Similar failures in Flaky Test Issue - #14331

---

For org.opensearch.index.ShardIndexingPressureSettingsIT.testShardIndexingPressureLastSuccessfulSettingsUpdate
Flaky Test - #14328

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✅ Gradle check result for fa2373f: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 81.63265% with 18 lines in your changes missing coverage. Please review.

Project coverage is 71.83%. Comparing base (acee2ae) to head (40e0fc9).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
.../java/org/opensearch/indices/NodeIndicesStats.java	85.71%	7 Missing and 5 partials ⚠️
...in/java/org/opensearch/indices/IndicesService.java	25.00%	2 Missing and 1 partial ⚠️
...h/action/admin/indices/stats/CommonStatsFlags.java	75.00%	0 Missing and 2 partials ⚠️
...rg/opensearch/rest/action/cat/RestNodesAction.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #14454      +/-   ##
============================================
- Coverage     71.87%   71.83%   -0.04%     
- Complexity    63318    63415      +97     
============================================
  Files          5231     5244      +13     
  Lines        296521   296864     +343     
  Branches      42832    42868      +36     
============================================
+ Hits         213113   213250     +137     
- Misses        65948    66097     +149     
- Partials      17460    17517      +57     

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