Changes to support unmapped fields in metric aggregation

[expani]

Description

Handling empty field context for fields absent in the index.

Related Issues

Resolves #16473

[github-actions]

❌ Gradle check result for 1f8dcc6: 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]

@expani Thanks for debugging this. Can you please add a unit test case and move this out of draft state.

[github-actions]

✅ Gradle check result for 51eb573: SUCCESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.22%. Comparing base (ab546ae) to head (a7202c0).
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #16481      +/-   ##
============================================
- Coverage     72.23%   72.22%   -0.01%     
- Complexity    65210    65287      +77     
============================================
  Files          5299     5299              
  Lines        303475   303475              
  Branches      43925    43925              
============================================
- Hits         219217   219193      -24     
- Misses        66254    66365     +111     
+ Partials      18004    17917      -87     

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[github-actions]

❌ Gradle check result for a1256aa: FAILURE

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[github-actions]

❌ Gradle check result for 47b2996: FAILURE

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❌ Gradle check result for f2d0091: FAILURE

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[github-actions]

❌ Gradle check result for b3c736b: FAILURE

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[expani]

@sandeshkr419 @bharath-techie I have added a unit test for this bug in MetricAggregatorTest and it's failing when removing the fix with the same NullPointerException

./gradlew ':server:test' --tests "org.opensearch.search.aggregations.startree.MetricAggregatorTests.*" -Druntime.java=21

Kindly review the PR.

[expani]

Looks like merging from main is making the UTs fail again. Taking a look

[github-actions]

❌ Gradle check result for 7a231e8: FAILURE

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[github-actions]

❌ Gradle check result for ea6929e: FAILURE

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[github-actions]

✅ Gradle check result for a0b9e48: SUCCESS

[github-actions]

❌ Gradle check result for 330088c: FAILURE

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[github-actions]

✅ Gradle check result for 6432da4: SUCCESS

[github-actions]

❌ Gradle check result for 6bf70e4: FAILURE

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[github-actions]

✅ Gradle check result for a7202c0: SUCCESS

[expani]

The UTs are passing now. @bharath-techie @msfroh @sandeshkr419 Please review when you get sometime.

[bharath-techie]

Thanks for the changes LGTM