Add TLS upstream and TLS server_zone metrics

[Dean-Coakley]

Proposed changes

• Add TLS upstream metrics
• Add TLS server_zone metrics.
• Fix unit test delta coverage

Note: R27+

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: 79.54% and project coverage change: -0.47% ⚠️

Comparison is base (1e92813) 66.83% compared to head (76d224a) 66.36%.
Report is 7 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #470      +/-   ##
==========================================
- Coverage   66.83%   66.36%   -0.47%     
==========================================
  Files         113      118       +5     
  Lines       12931    13313     +382     
==========================================
+ Hits         8642     8835     +193     
- Misses       3719     3892     +173     
- Partials      570      586      +16     

Files Changed	Coverage Δ
...or/github.com/nginx/agent/sdk/v2/config_helpers.go	64.19% <ø> (ø)
...nx/agent/v2/src/core/metrics/sources/nginx_plus.go	70.17% <76.31%> (-0.14%)	⬇️
...om/nginx/agent/v2/src/core/metrics/metrics_util.go	82.74% <100.00%> (+0.30%)	⬆️

... and 16 files with indirect coverage changes

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