Update setting API honors cluster's replica setting as default #14810

[chishui]

Description

This is a fix to #14810.
When user calls

PUT index/_settings
{
  "index": {
    "number_of_replicas": null
  }
}

Previous logic is always set index.number_of_replicas to 1. This PR changes the default value to the one configured by cluster setting cluster.default_number_of_replicas and if it's not configured, a same default 1 will be used. It's consistent to the default replica value when creating an index.

Related Issues

#14810

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 f9e019f: UNSTABLE

• TEST FAILURES:

      2 org.opensearch.common.util.concurrent.QueueResizableOpenSearchThreadPoolExecutorTests.classMethod
      1 org.opensearch.common.util.concurrent.QueueResizableOpenSearchThreadPoolExecutorTests.testResizeQueueDown

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[github-actions]

❕ Gradle check result for a7d986e: 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 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 71.95%. Comparing base (036f6bc) to head (9286a1b).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...luster/metadata/MetadataUpdateSettingsService.java	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #14948      +/-   ##
============================================
+ Coverage     71.92%   71.95%   +0.03%     
- Complexity    64400    64413      +13     
============================================
  Files          5281     5281              
  Lines        300995   300996       +1     
  Branches      43479    43479              
============================================
+ Hits         216491   216590      +99     
+ Misses        66793    66598     -195     
- Partials      17711    17808      +97     

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[github-actions]

❌ Gradle check result for b690c89:

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[github-actions]

❌ Gradle check result for e16d52f: FAILURE

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[github-actions]

❌ Gradle check result for 157b40c: FAILURE

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[github-actions]

❌ Gradle check result for 46ba1fe: FAILURE

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[gaobinlong]

This change looks good to me, please resolve the conflicts and make sure all gradle checks pass, thanks!

[github-actions]

❌ Gradle check result for e6b8976: FAILURE

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[github-actions]

❌ Gradle check result for 238b01e: FAILURE

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[github-actions]

❌ Gradle check result for 21e4a19: FAILURE

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[chishui]

❌ Gradle check result for 21e4a19: FAILURE

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#16015

[github-actions]

❕ Gradle check result for 9286a1b: UNSTABLE

• TEST FAILURES:

      1 org.opensearch.snapshots.DedicatedClusterSnapshotRestoreIT.testSnapshotWithStuckNode

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[chishui]

This change looks good to me, please resolve the conflicts and make sure all gradle checks pass, thanks!

@gaobinlong I resolved conflict and all gradle checks passed

[gaobinlong]

@chishui could you open a documentation PR about this behavior in this repo? Maybe add it here: dynamic-index-level-index-settings, and we even don't have some documentation about the cluster level setting cluster.default_number_of_replicas, could you also help to add it here: cluster-settings.

[gaobinlong]

@reta @andrross @dblock mind taking a second look at this PR?