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]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lhy1024
• rleungx

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c7b5055) 74.76% compared to head (ac9155f) 74.63%.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-7.1    #7829      +/-   ##
===============================================
- Coverage        74.76%   74.63%   -0.14%     
===============================================
  Files              411      411              
  Lines            41716    41722       +6     
===============================================
- Hits             31191    31139      -52     
- Misses            7785     7832      +47     
- Partials          2740     2751      +11     

Flag	Coverage Δ
unittests	74.63% <100.00%> (-0.14%)	⬇️

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[lhy1024]

/merge

[ti-chi-bot]

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[ti-chi-bot]

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

Commit hash: ac9155f