statistics: fix wrong singleflight implementation for stats' syncload(#52301) (#52382)

[winoros]

cherry pick #52382

close #52294

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

What changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AilinKid, hawkingrei

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-28 12:22:33.604528153 +0000 UTC m=+959348.738978262: ☑️ agreed by hawkingrei.
• 2024-08-28 12:26:03.20386484 +0000 UTC m=+959558.338314986: ☑️ agreed by AilinKid.