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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[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 02:56:17.927409333 +0000 UTC m=+670647.667833272: ☑️ agreed by rleungx.
• 2024-09-14 03:16:04.868236541 +0000 UTC m=+671834.608660472: ☑️ agreed by HuSharp.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.61%. Comparing base (41c6bc2) to head (f7a3280).
Report is 1 commits behind head on release-5.4.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-5.4    #7826      +/-   ##
===============================================
+ Coverage        74.57%   74.61%   +0.03%     
===============================================
  Files              265      265              
  Lines            28028    28048      +20     
===============================================
+ Hits             20903    20928      +25     
+ Misses            5264     5256       -8     
- Partials          1861     1864       +3     

Flag	Coverage Δ
unittests	74.61% <100.00%> (+0.03%)	⬆️

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