Add getMetadataFields to MapperService

[cwperks]

Description

This PR adds a method to the MapperService to get the full list of metadata fields.

This is needed in the security plugin to remove dependence on a static list maintained here: https://github.com/opensearch-project/security/blob/main/src/main/java/org/opensearch/security/configuration/SecurityFlsDlsIndexSearcherWrapper.java#L41-L59

The primary list is maintained in IndicesModules.getBuiltInMetadataFields, but this class is marked as @internal and not meant to be referenced by plugins.

Related Issues

Related to opensearch-project/security#4349

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• API changes companion pull request created.
• Failing checks are inspected and point to the corresponding known issue(s) (See: Troubleshooting Failing Builds)
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)
• Public documentation issue/PR created

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

✅ Gradle check result for 622dc08: SUCCESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.62%. Comparing base (b15cb0c) to head (b5cbb78).
Report is 306 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #13819      +/-   ##
============================================
+ Coverage     71.42%   71.62%   +0.20%     
- Complexity    59978    61300    +1322     
============================================
  Files          4985     5063      +78     
  Lines        282275   288028    +5753     
  Branches      40946    41710     +764     
============================================
+ Hits         201603   206294    +4691     
- Misses        63999    64789     +790     
- Partials      16673    16945     +272     

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[github-actions]

❌ Gradle check result for e404fa9: 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 b5cbb78: SUCCESS

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch/backport-2.x
# Create a new branch
git switch --create backport/backport-13819-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 8883e62a8f3d862739250cff1eb8d3d18680cfa2
# Push it to GitHub
git push --set-upstream origin backport/backport-13819-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-13819-to-2.x.

[reta]

@cwperks mind please backport to 2.x manually? thank you