Add grpc integration tests

[dhurley]

Proposed changes

Add grpc integration tests

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: 89.47% and project coverage change: -0.20% ⚠️

Comparison is base (dbfd11e) 66.40% compared to head (672bcf6) 66.21%.
Report is 1 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #475      +/-   ##
==========================================
- Coverage   66.40%   66.21%   -0.20%     
==========================================
  Files         118      118              
  Lines       13398    13384      -14     
==========================================
- Hits         8897     8862      -35     
- Misses       3911     3932      +21     
  Partials      590      590              

Files Changed	Coverage Δ
...dor/github.com/nginx/agent/v2/src/plugins/nginx.go	71.63% <50.00%> (-0.57%)	⬇️
.../github.com/nginx/agent/v2/src/plugins/features.go	37.00% <87.50%> (+0.27%)	⬆️
...or/github.com/nginx/agent/v2/src/plugins/common.go	70.23% <100.00%> (ø)
...or/github.com/nginx/agent/v2/src/plugins/events.go	84.17% <100.00%> (ø)

... and 4 files with indirect coverage changes

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