[Backport 2.x] Retrieve value from DocValues in a flat_object filed

[kkewwei]

Description

Backport cf49717 from #16802

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

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

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 6dfbd0f: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 56.00000% with 11 lines in your changes missing coverage. Please review.

Project coverage is 71.96%. Comparing base (421f211) to head (6dfbd0f).
Report is 1 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...opensearch/index/mapper/FlatObjectFieldMapper.java	54.54%	7 Missing and 3 partials ⚠️
...a/org/opensearch/index/mapper/DocValueFetcher.java	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #17106      +/-   ##
============================================
+ Coverage     71.90%   71.96%   +0.05%     
- Complexity    65701    65729      +28     
============================================
  Files          5322     5322              
  Lines        306191   306214      +23     
  Branches      44650    44655       +5     
============================================
+ Hits         220177   220363     +186     
+ Misses        67668    67473     -195     
- Partials      18346    18378      +32     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.