Wait grace period seconds after pod creation/upgrade when traffic rolling

[veophi]

Ⅰ. Describe what this PR does

Ⅱ. Does this pull request fix one issue?

Ⅲ. Special notes for reviews

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (07b7f20) 43.56% compared to head (5bb8ee0) 43.59%.

Files	Patch %	Lines
pkg/trafficrouting/manager.go	87.50%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #185      +/-   ##
==========================================
+ Coverage   43.56%   43.59%   +0.03%     
==========================================
  Files          52       52              
  Lines        5672     5677       +5     
==========================================
+ Hits         2471     2475       +4     
- Misses       2778     2779       +1     
  Partials      423      423              

Flag	Coverage Δ
unittests	43.59% <87.50%> (+0.03%)	⬆️

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[zmberg]

/lgtm

[zmberg]

/approve

[kruise-bot]

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This pull-request has been approved by: zmberg

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