Task: Prometheus Metrics - Update

[jonrandahl]

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

Definition of Done

• Add Puma-metrics Gem [^1]
• [x] Update config/puma.rb to include metrics plugin and port info
• [x] Update config/initializers/prometheus.rb to add Middleware instrumentation
• [x] Update config.ru to ensure prometheus is not triggered in the test environment
• [x] Update app/subscribers/action_controller_prometheus_subscriber.rb
• [x] Update the prometheus metrics notification to the application exceptions controller^2
• [x] Update the puma-stats to display in the footer while in development
• [x] Add the Prometheus monitoring information to the project README/WIKI

[^1]: No Puma Metrics due to gem version requirement conflict

[jonrandahl]

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