Add server zone metrics

[sjberman]

Problem: server_zone metrics were not being emitted for NGINX Plus users

Solution: Add these metrics to the server blocks when running NGINX Plus

Testing: Verified that server_zone metrics show up in Prometheus

Closes #2220

Checklist

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

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

Added server_zone metrics for NGINX Plus users.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.85%. Comparing base (6289c6e) to head (17eef72).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2360   +/-   ##
=======================================
  Coverage   88.85%   88.85%           
=======================================
  Files         100      100           
  Lines        7509     7513    +4     
  Branches       50       50           
=======================================
+ Hits         6672     6676    +4     
  Misses        781      781           
  Partials       56       56           

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