ticdc/mq: accurately demonstrate txn_batch_size metric for MQ sink (#3609)

[ti-chi-bot]

This is an automated cherry-pick of #3609

What problem does this PR solve?

#3431

What is changed and how it works?

In order to accurately show the txn_batch_size metric, we use the total number of rows instead of messages when the sink target is MQ.

Check List

Tests

• Unit test
• Integration test

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has not been approved.

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-5.0@7255fc7). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff                @@
##             release-5.0      #3820   +/-   ##
================================================
  Coverage               ?   57.2304%           
================================================
  Files                  ?        167           
  Lines                  ?      19729           
  Branches               ?          0           
================================================
  Hits                   ?      11291           
  Misses                 ?       7365           
  Partials               ?       1073           

[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: b38f584

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@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.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

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.