Add metric sender feature

[Dean-Coakley]

Proposed changes

• Add metric-sender feature so it is now possible to enable/disable as desired
• Add metric-collection feature
• Add metric feature which includes metrics-collection, metrics-throttle and metrics-sender features
• Improve metric plugin can so it can now stream metric reports directly to metric-sender
• Linting

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: 25.53% and project coverage change: -0.39% ⚠️

Comparison is base (7c1ada3) 67.19% compared to head (b0ea6a7) 66.81%.
Report is 1 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #453      +/-   ##
==========================================
- Coverage   67.19%   66.81%   -0.39%     
==========================================
  Files         113      113              
  Lines       12849    12912      +63     
==========================================
- Hits         8634     8627       -7     
- Misses       3647     3717      +70     
  Partials      568      568              

Files Changed	Coverage Δ
...b.com/nginx/agent/v2/src/plugins/metrics_sender.go	59.42% <0.00%> (ø)
.../nginx/agent/sdk/v2/agent/config/config_helpers.go	28.20% <ø> (-1.07%)	⬇️
...or/github.com/nginx/agent/sdk/v2/config_helpers.go	64.00% <0.00%> (ø)
...r/github.com/nginx/agent/v2/src/plugins/metrics.go	66.92% <11.11%> (-3.49%)	⬇️
.../github.com/nginx/agent/v2/src/plugins/features.go	35.02% <28.57%> (-1.54%)	⬇️
...om/nginx/agent/v2/src/plugins/metrics_throlling.go	58.46% <50.00%> (ø)
...ginx/agent/v2/src/core/metrics/collectors/nginx.go	87.35% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[oliveromahony]

Tested and LGTM