Add worker conn metrics

[Dean-Coakley]

Proposed changes

• Add worker metrics from R30 release
• Change: detect latest api version and use that rather than defaulting to 6.
  • Now falls back to version 7 if apiVersion can not be determined
• Improve nginx plus collector tests. Reduced footprint and increase coverage

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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[codecov-commenter]

Codecov Report

Patch coverage: 74.11% and project coverage change: +0.06% 🎉

Comparison is base (a9728fe) 66.19% compared to head (dabeaff) 66.26%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #461      +/-   ##
==========================================
+ Coverage   66.19%   66.26%   +0.06%     
==========================================
  Files         118      118              
  Lines       13313    13390      +77     
==========================================
+ Hits         8813     8873      +60     
- Misses       3916     3927      +11     
- Partials      584      590       +6     

Files Changed	Coverage Δ
src/core/config/defaults.go	0.00% <ø> (ø)
...nx/agent/v2/src/core/metrics/sources/nginx_plus.go	70.57% <72.15%> (+0.39%)	⬆️
...om/nginx/agent/v2/src/core/metrics/metrics_util.go	83.04% <100.00%> (+0.29%)	⬆️

... and 2 files with indirect coverage changes

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