Update NGINX plugin to read NGINX config on startup

[dhurley]

Proposed changes

• Update nginx-agent.conf for grafana example
• Update NGINX plugin to read NGINX config on startup to get access/error log information
• Add fix do that Agent start and NGINX found events get sent

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

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (df8cf6e) 66.29% compared to head (0def6fc) 66.28%.
Report is 4 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #489      +/-   ##
==========================================
- Coverage   66.29%   66.28%   -0.01%     
==========================================
  Files         118      118              
  Lines       13441    13446       +5     
==========================================
+ Hits         8911     8913       +2     
- Misses       3936     3938       +2     
- Partials      594      595       +1     

Files	Coverage Δ
src/core/pipe.go	54.09% <50.00%> (ø)
...dor/github.com/nginx/agent/v2/src/plugins/nginx.go	71.15% <75.00%> (-0.28%)	⬇️

... and 3 files with indirect coverage changes

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