util: improve TopSQL reporter memory efficiency (#25195)

[ti-srebot]

cherry-pick #25195 to release-5.1
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/25363

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/25363:release-5.1-f4e608b8ba96

---

What problem does this PR solve?

Problem Summary:

The current TopSQL reporter copies data before sending to gRPC endpoint.

What is changed and how it works?

What's Changed:
Iterate data at send time to reduce a memory copy.

How it Works:
Because the SQL/plan meta could be > 1M for each record, this could potentially save some GB of memory.

Related changes

Check List

Tests

• Unit test
• Integration test

Side effects

None.

Release note

• No release note

[ti-srebot]

/run-all-tests

[ti-srebot]

@dragonly please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/tidb/invitations

[crazycs520]

LGTM

[crazycs520]

/run-all-tests

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• breeswish
• crazycs520

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[breezewish]

/merge

[ti-chi-bot]

@breeswish: /merge is only allowed for the committers, you can assign this pull request to the committer in list by filling /assign @committer in the comment to help merge this pull request.

In response to this:

/merge

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[breezewish]

@zhouqiang-cl Please help run merge, thanks a lot!

[zhouqiang-cl]

/merge

[ti-chi-bot]

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

Commit hash: c13076c

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.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

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[zhouqiang-cl]

/run-unit-test