add cache metrics for NGINX plus

[sheharyaar]

Proposed changes

Fixes #522 and exposes cache metrics from NGINX Plus API.

Checklist

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

• I have read the CONTRIBUTING
  guide
• I have proven my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have ensured the README is up to date
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch on my own fork

[sheharyaar]

The lint check has failed, do I need to perform gofumt in my branch or is it fine ?

[jjngx]

The lint check has failed, do I need to perform gofumt in my branch or is it fine ?

Yes, please do run gofumpt.

[sheharyaar]

Done, fixed lint issues and rebased

[shaun-nx]

Hi @sheharyaar, thanks for contributing to this project! We can merge this PR as soon as the lint errors are fixed.

I get these two errors when running make lint locally

collector/nginx_plus.go:1210: File is not `gofumpt`-ed (gofumpt)

collector/nginx_plus.go:1190:97: unnecessary conversion (unconvert)
		ch <- prometheus.MustNewConstMetric(c.cacheZoneMetrics["cold"], prometheus.GaugeValue, float64(cold), name)

For the first error, just run gomt -w collector/nginx_plus.go

For the 2nd error, it seems like the float64(cold) conversion isn't needed.
I would remove this conversation and test if the metric still displays as you expect.

[shaun-nx]

@sheharyaar ignore my last command, I was late to the trigger 😆

[shaun-nx]

Thank you @sheharyaar ! 🎉

[sheharyaar]

Gald to contribute, welcome !