Task: Prometheus Metrics - Update

[jonrandahl]

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

Definition of Done

• [x] Add Puma-metrics Gem
• [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^1
• [x] Update the puma-stats to display in the footer while in development [^2]
• [x] Add the Prometheus monitoring information to the project README/WIKI

[^2]: Also, due to the Landing page, PPD, and Standard Reports loading their templates from the LR Common Styles gem, the changes for the puma-stats to be displayed in the footer while in development have required a new gem version and release: v1.9.9

[jonrandahl]

As this repository has had the information previously held in the README moved to the WIKI, the prometheus monitoring information is now here:

https://github.com/epimorphics/standard-reports-ui/wiki#prometheus-monitoring