Add Pitchfork/Raindrops middleware

[robertomiranda]

Add Pitchfork/Raindrops middleware. This PR depends on the changes made in #63. However, it removes the Periodic Stats class introduced previously, considering the feedback on the previous PR. We can periodically collect metrics, relying on the health checks performed on the Rails application.

Note: I'm not testing the changes made to Railties because they are too complex and add little value. Instead, I'm proposing to ignore it

Promenade::Raindrops::Middleware

The Promenade::Raindrops::Middleware middleware collects and reports metrics fromPromenade::Raindrops::Stats. The Promenade::Raindrops::Stats exposes active workers and queued requests in servers like Rainbows, Unicorn, and Pitchfork using Prometheus gauges. It initializes by obtaining TCP listener stats from Raindrops::Linux, setting the listener address, and fetching active worker and queued request counts. The instrument method updates the Prometheus gauges with these values, and the class method self.instrument allows easy instrumentation with an optional listener address.

Promenade::Raindrops::Stats.instrument(listener_address: name)
end

• rack_active_workers: Number of active workers in the Application Server
• rack_queued_requests: Number of requests waiting to be processed by the Application Server

Promenade::Pitchfork::Middleware

The Promenade::Pitchfork::Middleware class collects and reports metrics using the Promenade::Pitchfork::Stats class. The Promenade::Pitchfork::Stats exports the Pitchfork worker processes using Prometheus gauges. It tracks the total, live, idle, and busy worker counts. The class retrieves these metrics using Pitchfork::Info and Raindrops::Stats, and updates the Prometheus gauges accordingly.

Promenade::Pitchfork::Stats.instrument

• pitchfork_workers_count: Total number of configured workers.
• pitchfork_live_workers_count: Number of live (booted) workers.
• pitchfork_capacity: Number of idle workers.
• pitchfork_busy_percent: Percentage of busy workers.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (328463b) to head (426a6f3).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #66   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           34        37    +3     
  Lines          761       833   +72     
=========================================
+ Hits           761       833   +72     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.