Refactor IndexingStats.Stats with Builder pattern

[pado0]

Description

This PR refactors the IndexingStats.Stats class to use the Builder pattern instead of relying on multiple constructors.

By adopting the Builder pattern, it becomes easier to evolve the stats API, add new metrics, and maintain backward compatibility without forcing disruptive constructor changes.

Based on the related issue:

1. Added a Builder and deprecated the existing constructors.
2. Replaced usages of constructors in code and tests with the new Builder.

There are multiple stats-related classes that need similar refactoring, and we are addressing them in priority order. This PR covers IndexingStats.Stats as part of that effort.

Related Issues

Partially resolves #19225

Check List

• Functionality includes testing.

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[github-actions]

❌ Gradle check result for e7609b4: 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?

[sandeshkr419]

Thanks @pado0 for working on the changes. The changes look good overall, few comments:

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I see you ran into spotless (indenting) failures.
Run ./gradlew :server:spotlessApply to fix these violations and then commit the changes again.

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Add a changelog. Refer: https://github.com/opensearch-project/OpenSearch/blob/main/CONTRIBUTING.md

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nit: mentioning Resolves <any issue> closes the issue once the PR is merged. For changes which are partially resolving the issue, mention Partially resolves <any issue> to keep the primary issue open. Not something to worry about but I thought to mention it anyway.

[github-actions]

✅ Gradle check result for f9c10a9: SUCCESS

[github-actions]

✅ Gradle check result for 3e3fddd: SUCCESS

[github-actions]

❌ Gradle check result for 2c958be: 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 2c958be: 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 b10ecf2: FAILURE

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[sandeshkr419]

@peterzhuamazon @gaiksaya Is there a pending change in main causing builds to break?

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':distribution:bwc:minor:buildBwcLinuxTar'.
> Building 3.3.0 didn't generate expected file /var/jenkins/workspace/gradle-check/search/distribution/bwc/minor/build/bwc/checkout-3.3/distribution/archives/linux-tar/build/distributions/opensearch-min-3.3.0-SNAPSHOT-linux-x64.tar.gz

[github-actions]

❌ Gradle check result for b10ecf2: 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 1d6a88d: 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 3ef8c9a: SUCCESS