Prometheus exporter integration

[cgrard]

Description

First I'd like to thank you for this amazing app, it is really very useful and very well done.
I've been using PasswordPusher for quite a while (everyone loves it) and it is currently deployed in a Kubernetes environnement where I felt the need to improve monitoring and observability lately, so I wondered if there was some kind of /metrics exposed somehow and found out that there isn't. I haven't worked with Ruby in years, and it has never been my primary language, so I apologize in advance if the code is not of the highest quality, I did my best.

I will probably also provide a Grafana Dashboard at some point if anyone is interested as it is a pretty popular observability ecosystem.

TL;DR This PR adds comprehensive Prometheus metrics export functionality to Password Pusher, enabling monitoring, observability, and security analytics.

Features implemented:

• Native Prometheus exporter integration using prometheus_exporter gem
• Automatic startup via Procfile (development and production)
• 14 custom business metrics tracking:
  • Push lifecycle (creation, views, expiration)
  • Security events (failed views, failed passphrase attempts)
  • Admin & audit (admin views, owner views)
  • File uploads (count and bytes)
  • User authentication (signup, login, logout, lockout)
• Standard Rails metrics (HTTP requests, database, process, Puma)
• Comprehensive documentation with PromQL examples, dashboards, and alerts

Architecture:

• Standalone Prometheus exporter server on port 9394
• Rails middleware and ActiveRecord callbacks for automatic metric tracking
• Graceful shutdown handling
• Environment-aware (disabled in test, Puma metrics only in production)

Related Issue

Neither related issue nor implementation request

Type of Change

• 📚 Examples / docs / tutorials / dependencies update
• 🔧 Bug fix (non-breaking change which fixes an issue)
• 🥂 Improvement (non-breaking change which improves an existing feature)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🔐 Security fix

Checklist

• I've written tests (if applicable) for all new methods and classes that I created. (rake test)
• I've added documentation as necessary so users can easily use and understand this feature/fix.

Additional Notes

New files:

• config/prometheus_server.rb - Prometheus exporter server with custom collectors
• config/initializers/prometheus.rb - Client configuration and instrumentation
• config/initializers/devise_prometheus.rb - Devise authentication event tracking
• app/models/concerns/prometheus_metrics.rb - Reusable metrics tracking concern
• test/prometheus_test.rb - Tests
• PROMETHEUS.md - Setup and deployment documentation
• METRICS.md - Complete metrics reference (600+ lines with queries, dashboards, alerts)

Not sure about the .md documentation, I think that both files should be converted into a specific chapter in the documentation and be removed from the repo, don't they?

Modified files:

• Gemfile - Added prometheus_exporter gem
• Procfile / Procfile.dev - Added prometheus process
• app/models/push.rb - Added metrics tracking callbacks
• app/models/audit_log.rb - Added view and security metrics tracking
• app/models/user.rb - Added authentication metrics tracking

**Testing:** Tests have not been written for this PR as: - Prometheus metrics are disabled in the test environment (`Rails.env.test?`) - The feature is non-invasive and fails gracefully if the exporter is unavailable - Comprehensive manual testing has been performed in development environment - Metrics tracking uses simple ActiveRecord callbacks without complex logic

Tests could be added in a follow-up PR if desired, potentially mocking the PrometheusExporter::Client to verify correct metric tracking.

What do you think?

[github-actions]

Hello @cgrard, thank you for submitting a PR! We will respond as soon as possible.

[pglombardo]

Hi @cgrard - this is a great idea - thanks for creating! I'm familiar with Prometheus but haven't used it in a few years. I'll take a closer look very soon.

[cgrard]

Hence the importance of unit tests ! It turns out I actually broke something by modifying the scope without realizing it, but it has now been fixed and all tests pass successfully on a rake test

[pglombardo]

Hi @cgrard - just a heads up that I haven't forgotten about this PR. I want to push to merge this sometime soon. Thanks for the patience!