Switch percentiles implementation to MergingDigest

[peteralfonsi]

Description

Switches the percentiles agg implementation from AVLTreeDigest to MergingDigest. MergingDigest is now the recommended implementation from the t-digest library, and we see significant speedups by switching to it. (See more details in the original issue). Some latencies from percentiles aggregations on http_logs:

Field	Baseline latency (ms)	Modififed latency (ms)
timestamp	13,085	6,293
status	196,794	6,212

Since the implementations extend the same abstract class AbstractTDigest, this is a drag-and-drop change. Our extending class TDigestState just adds some StreamInput/StreamOutput methods, which use the result of the centroids() and centroidCount() methods which are overridden from the abstract class. I don't think we need additional tests. The existing tests in TDigestPercentilesAggregatorTests.java still pass after the change. However I will still mark this PR as a draft, please let me know if I'm missing something here...

I also don't think we need any tuning of the sole parameter compression, since in the library's method createDigest that creates a digest of the default implementation, it doesn't modify the input compression based on which implementation it's actually using.

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 852c178: 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]

On further thought I will add some tests around serialization + serialization inter-version compatibility. Both classes provide their own serialization methods. Probably the most logical thing to do would be to add a version check, and for < 3.1 use the preexisting logic assuming TDigestState extends AVLTreeDigest, and for >= 3.1 just rely on the library's serialization code.

[github-actions]

❌ Gradle check result for 4599cff: 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 e4caace: SUCCESS