server: add more log for drainClients (DNM)

[YangKeao]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

This is the debug branch for #55494

What changed and how does it work?

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.

None

[ti-chi-bot]

[FORMAT CHECKER NOTIFICATION]

Notice: To remove the do-not-merge/needs-linked-issue label, please provide the linked issue number on one line in the PR body, for example: Issue Number: close #123 or Issue Number: ref #456.

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Tests

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

_{📖 For more info, you can check the "Contribute Code" section in the development guide.}

[ti-chi-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 75.1338%. Comparing base (2045418) to head (69f0ee7).
Report is 38 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #55516        +/-   ##
================================================
+ Coverage   73.0142%   75.1338%   +2.1195%     
================================================
  Files          1576       1577         +1     
  Lines        440774     443133      +2359     
================================================
+ Hits         321828     332943     +11115     
+ Misses        99264      89999      -9265     
- Partials      19682      20191       +509     

Flag	Coverage Δ
integration	49.1596% <14.2857%> (?)
unit	71.8704% <57.1428%> (-0.2266%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	52.9567% <ø> (ø)
parser	∅ <ø> (∅)
br	52.5330% <ø> (+6.5358%)	⬆️