statistics: support merge global topn in concurrency (#38358)

[ti-chi-bot]

This is an automated cherry-pick of #38358

What problem does this PR solve?

Issue Number: ref #35142

Problem Summary:

merging global topn stats is time consuming

What is changed and how it works?

This pr makes it running in concurrency

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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