dxf: fix the task num count incorrectly (#57127)

[ti-chi-bot]

This is an automated cherry-pick of #57127

What problem does this PR solve?

Issue Number: close #57172

Problem Summary:

What changed and how does it work?

The task count metric is updating in scheduler loop, but the task maybe cleaned up in doCleanupTask(), the scheduler will not handle the finished/failed... state, so the task count metric will not be reduced.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  Use sysbench prepare 100 tables with 10000 rows. Watch the pannel:

• 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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release-8.5@f485d63). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.5     #57386   +/-   ##
================================================
  Coverage               ?   56.8451%           
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  Files                  ?       1770           
  Lines                  ?     625668           
  Branches               ?          0           
================================================
  Hits                   ?     355662           
  Misses                 ?     245975           
  Partials               ?      24031           

Flag	Coverage Δ
integration	37.0460% <100.0000%> (?)
unit	72.5249% <100.0000%> (?)

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

Components	Coverage Δ
dumpling	52.9478% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	52.3943% <0.0000%> (?)

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fzzf678, tangenta

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• OWNERS [tangenta]
• pkg/disttask/OWNERS [tangenta]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-11-14 10:07:23.462660543 +0000 UTC m=+523605.653529541: ☑️ agreed by fzzf678.
• 2024-11-14 10:10:45.830147655 +0000 UTC m=+523808.021016650: ☑️ agreed by tangenta.