Adding ScriptedAvg class to painless spi to allowlist usage from plugins

[tandonks]

Description

Adding ScriptedAvg class to painless spi to allowlist usage from plugins
Followup PR to: #18411

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.

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[github-actions]

❌ Gradle check result for f7ad412: FAILURE

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[github-actions]

❌ Gradle check result for b163b00: FAILURE

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[github-actions]

❌ Gradle check result for 4574641: FAILURE

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[github-actions]

✅ Gradle check result for 2108b6a: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.89%. Comparing base (10ff9d3) to head (e37dbd6).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
...a/org/opensearch/common/io/stream/Streamables.java	40.00%	3 Missing ⚠️
...earch/search/aggregations/metrics/InternalAvg.java	87.50%	0 Missing and 1 partial ⚠️
...earch/aggregations/metrics/InternalValueCount.java	85.71%	0 Missing and 1 partial ⚠️
...aggregations/metrics/ScriptedMetricAggregator.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19006      +/-   ##
============================================
- Coverage     72.94%   72.89%   -0.06%     
+ Complexity    69523    69440      -83     
============================================
  Files          5647     5647              
  Lines        319166   319175       +9     
  Branches      46165    46168       +3     
============================================
- Hits         232828   232653     -175     
- Misses        67539    67709     +170     
- Partials      18799    18813      +14     

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[github-actions]

❌ Gradle check result for b6161ea: 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 ba68f34: SUCCESS

[bharath-techie]

LGTM

[github-actions]

✅ Gradle check result for e37dbd6: SUCCESS