filter: refine comments and function visibility (#2308)

[ti-chi-bot]

This is an automated cherry-pick of #2308

What problem does this PR solve?

ref: https://github.com/pingcap/ticdc/pull/2218

What is changed and how it works?

Pick some code from the original PR.

Check List

Tests

• No code

Code changes

None

Side effects

None

Related changes

• Need to cherry-pick to the release branch

Release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

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[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

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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[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: c9c8580

[amyangfei]

/run-unit-tests