config: fix trace-region-flow displayed incorrectly

[rleungx]

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:

• CabinfeverB
• lhy1024

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[codecov]

Codecov Report

Merging #7993 (69593bb) into master (2c20897) will increase coverage by 0.06%.
The diff coverage is n/a.

❗ Current head 69593bb differs from pull request most recent head c90b783. Consider uploading reports for the commit c90b783 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7993      +/-   ##
==========================================
+ Coverage   73.60%   73.66%   +0.06%     
==========================================
  Files         436      436              
  Lines       48476    48474       -2     
==========================================
+ Hits        35679    35707      +28     
+ Misses       9739     9716      -23     
+ Partials     3058     3051       -7     

Flag	Coverage Δ
unittests	73.66% <ø> (+0.06%)	⬆️

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[rleungx]

/merge

[ti-chi-bot]

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[ti-chi-bot]

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

Commit hash: 69593bb

[ti-chi-bot]

@rleungx: Your PR was out of date, I have automatically updated it for you.

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[ti-chi-bot]

@rleungx: Your PR was out of date, I have automatically updated it for you.

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #8008.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #8064.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #8146.