metrics: set sync load latency as P9999 (#56981)

[ti-chi-bot]

This is an automated cherry-pick of #56981

What problem does this PR solve?

Issue Number: close #56980

Problem Summary:

What changed and how does it work?

The current sync load monitoring primarily focuses on P95 latency, which is insufficient for capturing extreme timeout issues. While P95 latency may not indicate a timeout, P9999 latency could be significantly high, leading to potential misjudgments and overlooked performance issues.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Enhanced sync load monitoring to include P9999 latency metrics for better detection and analysis of extreme performance issues

对 sync load 监控改为P9999，更好的观察性能问题

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release-8.1@ba10a69). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.1     #57001   +/-   ##
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Flag	Coverage Δ
unit	71.2110% <ø> (?)

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Components	Coverage Δ
dumpling	52.9656% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	41.3122% <0.0000%> (?)

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei, winoros

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-10-30 10:21:15.334707871 +0000 UTC m=+431588.173863414: ☑️ agreed by hawkingrei.
• 2024-10-30 12:59:15.060743994 +0000 UTC m=+441067.899899540: ☑️ agreed by winoros.