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]

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[3AceShowHand]

/run-all-tests

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-5.3@9779df7). Click here to learn what that means.
The diff coverage is n/a.

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[overvenus]

/merge

[ti-chi-bot]

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

Commit hash: 5637e19