config: fix trace-region-flow displayed incorrectly (#7993)

[ti-chi-bot]

This is an automated cherry-pick of #7993

What problem does this PR solve?

Issue Number: Close #7917.

What is changed and how does it work?

Check List

Tests

• Unit test
• Manual test

After this PR:

mysql> show config where name like "%pd-server.trace-region-flow%";
Empty set (0.01 sec)

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• rleungx

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.55%. Comparing base (f11bf75) to head (0c39e2f).

Additional details and impacted files

@@               Coverage Diff               @@
##           release-6.5    #8008      +/-   ##
===============================================
+ Coverage        75.44%   75.55%   +0.10%     
===============================================
  Files              332      332              
  Lines            33827    33825       -2     
===============================================
+ Hits             25522    25556      +34     
+ Misses            6096     6062      -34     
+ Partials          2209     2207       -2     

Flag	Coverage Δ
unittests	75.55% <ø> (+0.10%)	⬆️

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[JmPotato]

/merge

[ti-chi-bot]

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 0c39e2f