ticdc/token-region: don't set metric to 0 since multiple tables share it (#3480)

[ti-chi-bot]

This is an automated cherry-pick of #3480

What problem does this PR solve?

Metrics in token region are shared by multiple tables, so we should not use set to a set specific value

What is changed and how it works?

Use accurate inc or dec instead

Check List

Tests

• Unit test
• Integration test

Release note

Fix kv client cached region metric could be negative.

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-5.0@21d6d6f). Click here to learn what that means.
The diff coverage is n/a.

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[overvenus]

Cc #4300