Reapply "Switch percentiles implementation to MergingDigest (#18124)"

[peteralfonsi]

Description

Un-reverts #18124 which was previously reverted in advance of the 3.1 release in #18497. After discussion with @jainankitk and @msfroh we think it's safe to proceed with the faster MergingDigest implementation.

Also removes the test TDigestStateTests.testMoreThan4BValues. It was flaky after switching implementations (see discussion in #18476). But, its entire purpose was to check for regressions on this issue which is specific to the int[]/long[] array in AVLTreeDigest, so it isn't needed at all if we use MergingDigest and can be safely deleted.

Related Issues

Resolves #18122

Check List

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

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[github-actions]

❌ Gradle check result for a21f41d: 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?

[peteralfonsi]

Flaky test: #14294

[github-actions]

❌ Gradle check result for 471d6f8: 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?

[peteralfonsi]

Flaky test: #14294

[jainankitk]

Mostly LGTM, except for couple of comments!

[github-actions]

❌ Gradle check result for 3a46b83: FAILURE

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[github-actions]

❌ Gradle check result for 2bb35c8: FAILURE

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[peteralfonsi]

Flaky test: #18947

[github-actions]

❌ Gradle check result for da54be2: FAILURE

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[github-actions]

❕ Gradle check result for 2c6a6f1: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.12%. Comparing base (5468936) to head (2c6a6f1).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
...arch/search/aggregations/metrics/TDigestState.java	88.23%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19648      +/-   ##
============================================
+ Coverage     73.09%   73.12%   +0.03%     
- Complexity    70723    70775      +52     
============================================
  Files          5725     5725              
  Lines        323796   323819      +23     
  Branches      46886    46890       +4     
============================================
+ Hits         236673   236795     +122     
+ Misses        68009    67954      -55     
+ Partials      19114    19070      -44     

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