metrics(ticdc): remove meaningless checkpoint lag in slow table

[overvenus]

What problem does this PR solve?

Issue Number: ref #4757

What is changed and how it works?

Remove meaningless checkpoint lag in slow table.

Check List

Tests

• No code

Questions

Will it cause performance regression or break compatibility?

No

Do you need to update user documentation, design documentation or monitoring documentation?

No

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• 3AceShowHand
• sdojjy

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[overvenus]

/merge

[ti-chi-bot]

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

Commit hash: 25e7ba4

[ti-chi-bot]

@overvenus: Your PR was out of date, I have automatically updated it for you.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

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.

[overvenus]

/run-verify

[codecov-commenter]

Codecov Report

Merging #7413 (d02194f) into master (84ef430) will increase coverage by 0.1179%.
The diff coverage is 67.6356%.

Additional details and impacted files

Flag	Coverage Δ
cdc	67.6413% <74.3243%> (+0.1467%)	⬆️
dm	52.0282% <77.3584%> (+0.0570%)	⬆️
engine	61.0227% <35.4838%> (+0.1690%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

@@               Coverage Diff                @@
##             master      #7413        +/-   ##
================================================
+ Coverage   59.7990%   59.9170%   +0.1179%     
================================================
  Files           801        800         -1     
  Lines         91968      92099       +131     
================================================
+ Hits          54996      55183       +187     
+ Misses        32180      32124        -56     
  Partials       4792       4792