core: fix datarace in MergeLabels (#7537)

[ti-chi-bot]

This is an automated cherry-pick of #7537

What problem does this PR solve?

Issue Number: Close #7535

What is changed and how does it work?

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

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[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: HuSharp, rleungx

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-09-14 03:11:47.522187076 +0000 UTC m=+671577.262611015: ☑️ agreed by rleungx.
• 2024-09-14 03:15:37.288906226 +0000 UTC m=+671807.029330168: ☑️ agreed by HuSharp.

[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

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