etcdutil: refine the etcd client healthy checker code

[JmPotato]

What problem does this PR solve?

Issue Number: ref #7499.

What is changed and how does it work?

Refine the etcd client healthy checker code.

Check List

Tests

• Unit test
• Integration 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

Merging #7727 (d1f921e) into master (aa9c83c) will increase coverage by 0.05%.
The diff coverage is 88.13%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7727      +/-   ##
==========================================
+ Coverage   73.82%   73.87%   +0.05%     
==========================================
  Files         429      429              
  Lines       47528    47537       +9     
==========================================
+ Hits        35086    35118      +32     
+ Misses       9451     9441      -10     
+ Partials     2991     2978      -13     

Flag	Coverage Δ
unittests	73.87% <88.13%> (+0.05%)	⬆️

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[JmPotato]

/merge

[ti-chi-bot]

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[ti-chi-bot]

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

Commit hash: d1f921e