Fly.io is a platform that simplifies deploying and running Elixir applications by converting Docker containers into fleets of Firecracker micro-vms around the world. It also provides built-in Prometheus metrics for monitoring purposes. The integration with PromEx, an Elixir library, makes it easy to export various metrics from your Elixir applications and visualize them in Grafana dashboards. This combination allows developers to quickly set up application monitoring and proactively address issues without wasting time on extensive testing of all application components after each deployment.