[MINOR] fix(test): Fix flaky test for CoordinatorGrpcServerTest#testGrpcConnectionSize

[maobaolong]

What changes were proposed in this pull request?

Fix flaky test for CoordinatorGrpcServerTest#testGrpcConnectionSize

Why are the changes needed?

Do some efforts to make Ci more stable.

Does this PR introduce any user-facing change?

NO

How was this patch tested?

Existing UTs.

[github-actions]

Test Results

 2 966 files  ±0   2 966 suites  ±0   6h 30m 40s ⏱️ + 3m 22s
 1 100 tests ±0   1 098 ✅ ±0   2 💤 ±0  0 ❌ ±0 
13 774 runs  ±0  13 744 ✅ ±0  30 💤 ±0  0 ❌ ±0 

Results for commit 9e405f0. ± Comparison against base commit 3b0521a.

♻️ This comment has been updated with latest results.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.60%. Comparing base (580b4f2) to head (9e405f0).
Report is 6 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2323      +/-   ##
============================================
+ Coverage     51.19%   51.60%   +0.40%     
- Complexity     2936     2972      +36     
============================================
  Files           486      474      -12     
  Lines         24720    22683    -2037     
  Branches       2041     2090      +49     
============================================
- Hits          12656    11705     -951     
+ Misses        11274    10233    -1041     
+ Partials        790      745      -45     

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[jerqi]

LGTM.

[maobaolong]

@jerqi Thanks for your review! Merged.