In the 18th of July 2019 I gave a talk at Le Wagon Lisbon about how to instrument a Rails application, using Prometheus.
Since my last talk I grew fond of simple live coding demonstrations to convey practical knowledge as well as awareness that things are often easier than they seem. This time I went about gathering some system and custom metrics from an already existing application into an existing Prometheus server.
The application with the end setup is available here as a docker compose bundle that can easily be run to inspect results and do further experimentation. The instructions in the README should be clear enough to get you a working example, but if they are not then feel free to open an issue in the repository.