Add route and gateway metadata to nginx.http.requests

[NutsaB]

Proposed changes

Adds Kubernetes Gateway API route and gateway identity attributes (nginx.route.* and nginx.gateway.*) to the nginx.http.requests metric. Aggregates per-route request counts from access logs.
To populate route and gateway metadata, NGINX Gateway Fabric access logs must be written to a regular file on a shared volume rather than the default /dev/stdout destination.
Stub status scraper continues to emit global totals. If access log file destination is not configured, existing stub status metrics are uninterrupted.

Checklist

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• 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

[codecov]

Codecov Report

❌ Patch coverage is 97.61905% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.99%. Comparing base (d33070e) to head (9a301e3).

Files with missing lines	Patch %	Lines
...inxreceiver/internal/metadata/generated_metrics.go	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1971      +/-   ##
==========================================
- Coverage   89.00%   88.99%   -0.01%     
==========================================
  Files         111      111              
  Lines       12457    12489      +32     
==========================================
+ Hits        11087    11115      +28     
- Misses       1368     1372       +4     
  Partials        2        2              

Files with missing lines	Coverage Δ
...er/internal/scraper/accesslog/nginx_log_scraper.go	88.59% <100.00%> (+1.87%)	⬆️
...internal/scraper/stubstatus/stub_status_scraper.go	97.82% <100.00%> (ø)
...inxreceiver/internal/metadata/generated_metrics.go	92.55% <91.66%> (-0.09%)	⬇️

... and 1 file with indirect coverage changes

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[sjberman]

Hmm, I don't know if I like the fact that this is only possible by writing to a file. With lots of traffic, that file will get huge, fast.

Is this something we just aren't going to be able to do right now? It feels like we need a change to core nginx in order to add metadata to metrics.