Update NGINX Prometheus Exporter with systemd-activated socket

[huangsijun17]

Proposed changes

Add "Steps to add users"

Closes #799

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

---

Just a documentation update, validation code and test cases?

[nginx-bot]

Hi @huangsijun17! Welcome to the project! 🎉

Thanks for opening this pull request!
Be sure to check out our Contributing Guidelines while you wait for someone on the team to review this.

[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
_{Posted by the CLA Assistant Lite bot.}

[huangsijun17]

I have hereby read the F5 CLA and agree to its terms

[lucacome]

Thanks @huangsijun17 !