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]

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• JmPotato

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[JmPotato]

/merge

[ti-chi-bot]

@JmPotato: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

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

Commit hash: 785b57f