Revert "Remove deprecated constant META_FIELDS_BEFORE_7DOT8 (#13860)"

[cwperks]

This reverts commit 9832972.

Description

Opening a PR to discuss this reversion. Removing this deprecated constant caused issues in the SQL Plugin which references the constant. See opensearch-project/sql#2701 where they are working to adapt.

During Union operations, the SQL plugin relies on this list to differentiate between a metafield and a docfield. Ideally, the sql plugin can use MapperService.isMetadataField to determine from the official registry of mappers if a field is a metafield, but getting an instance of the mapper service within a plugin is proving to be tricky.

The security plugin uses Guice, to get the IndicesService:

1. GuiceHolder
2. getGuiceServiceClasses

Security can then get the indicesService using OpenSearchSecurityPlugin.GuiceHolder.getIndicesService() which then allows it to get a handle on the mapperService.

This PR is to temporarily fix SQL's CI issues from this change until a strategic fix is implemented.

For extra context around the fields in this list: (Copying comment from here)

7DOT8 is referring to ES 7.8.

"_ttl" and "_timestamp" were removed in ES6. See https://www.elastic.co/blog/elasticsearch-5-0-0-alpha4-released

"_type" was renamed to "_nested_path" in OS 2: #3196

"_size" is not a built-in metadata mapper, it comes from the mapper-size plugin which is not installed by default. Using mapperService.isMetadataField("_size") should return true if the mapper-size plugin is installed in the cluster.

The other values in this list still exist and are built-in metadata fields so mapperService.isMetadataField() should return true.

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

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[cwperks]

@ykmr1224 @fddattal I opened a revert PR for discussion. Please chime in with any additional context not already in the PR description.

[github-actions]

❌ Gradle check result for b59af56: 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 b59af56: SUCCESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.69%. Comparing base (b15cb0c) to head (b59af56).
Report is 329 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #13916      +/-   ##
============================================
+ Coverage     71.42%   71.69%   +0.27%     
- Complexity    59978    61358    +1380     
============================================
  Files          4985     5064      +79     
  Lines        282275   288113    +5838     
  Branches      40946    41719     +773     
============================================
+ Hits         201603   206571    +4968     
- Misses        63999    64474     +475     
- Partials      16673    17068     +395     

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