Task: Prometheus Metrics - Update

[jonrandahl]

Implement improvements from other applications and documented in the internal wiki:

Definition of Done

• Add Puma-metrics Gem ¹
• Update config/puma.rb to include metrics plugin and port info
• Update config/initializers/prometheus.rb to add Middleware instrumentation
• Update config.ru to ensure prometheus is not triggered in the test environment
• Update app/subscribers/action_controller_prometheus_subscriber.rb
• Update the prometheus metrics notification to the application exceptions controller²
• Update the puma-stats to display in the footer while in development
• Add the Prometheus monitoring information to the project README/WIKI

Footnotes

1. No Puma Metrics due to gem version requirement conflict ↩

2. https://github.com/epimorphics/front-end-templates/blob/main/prometheus-metrics/README.md#add-the-prometheus-metrics-to-the-application-exceptions-controller ↩

[jonrandahl]

N.B. While the ActiveSupport::Notification instrumentation is in place on the application controller the implementation is being updated in ticket #448