feat: Ignore ready and stats listener metrics in shutdown manager calculation

[zirain]

fixes: #7947

use listener.(.*).downstream_cx_active stats instead of server.total_connections which contains ready and stats listener.

[netlify]

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Name	Link
🔨 Latest commit	724b789
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[codecov]

Codecov Report

❌ Patch coverage is 63.41463% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.92%. Comparing base (a8952d0) to head (724b789).

Files with missing lines	Patch %	Lines
internal/cmd/envoy/shutdown_manager.go	63.41%	12 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7985      +/-   ##
==========================================
+ Coverage   72.86%   72.92%   +0.05%     
==========================================
  Files         237      237              
  Lines       35536    35550      +14     
==========================================
+ Hits        25894    25925      +31     
+ Misses       7801     7782      -19     
- Partials     1841     1843       +2     

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[arkodg]

can there be a case where resp is nil

[zirain]

AFAIK, resp shouldn't be nil when there's no error.