Introduce metrics representing N+ license status

[defanator]

Proposed changes

This PR adds N+ specific metrics indicating license status (expiration date plus some reporting counters available through the N+ API license object). Primary purpose of these is to be used in associated alerting, as N+ API is the lowest possible source of truth in regard to everything connected to N+ license.

The semantic is borrowed from similar cert-manager metrics which are widely used in lots of environments.

Example output in Prometheus format:

[2025-08-20 14:59:43 MDT] a.belov@F67QQWKXLK ~ % curl -fs http://localhost:9113/metrics | grep -i -- 'nginxplus_license'
# HELP nginxplus_license_expiration_timestamp_seconds License expiration date (expressed as Unix Epoch Time)
# TYPE nginxplus_license_expiration_timestamp_seconds gauge
nginxplus_license_expiration_timestamp_seconds 1.8220032e+09
# HELP nginxplus_license_reporting_fails_count Number of failed reporting attempts, reset each time the usage report is successfully sent
# TYPE nginxplus_license_reporting_fails_count gauge
nginxplus_license_reporting_fails_count 0
# HELP nginxplus_license_reporting_grace_period_seconds Number of seconds before traffic processing is stopped after unsuccessful report attempt
# TYPE nginxplus_license_reporting_grace_period_seconds gauge
nginxplus_license_reporting_grace_period_seconds 1.5551908e+07
# HELP nginxplus_license_reporting_healthy Indicates whether the reporting state is still considered healthy despite recent failed attempts
# TYPE nginxplus_license_reporting_healthy gauge
nginxplus_license_reporting_healthy 1

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

[nginx-bot]

Hi @defanator! 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.

[defanator]

@mpstefan @pdabelf5 @javorszky appreciate extra eyes on this one gentlemen, it would be great if it could land into next release of exporter itself + hopefully included into one of the next NIC releases. We have a few ongoing and upcoming tasks where we do consider lack of proposed metric as a blocker. TIA!

[Copilot]

Pull Request Overview

This PR introduces NGINX Plus license-related metrics to enable monitoring and alerting on license expiration and reporting status. The implementation adds a new API call to retrieve license information and exposes it through Prometheus metrics.

• Adds four new license-related metrics for expiration date, reporting health, failure count, and grace period
• Integrates with existing NGINX Plus client to fetch license data during metric collection
• Follows established patterns for metric naming and error handling

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