Check out the telemetry guide in the docs.

I second what @garrison said regarding telemetry for the Elixir side of things, but Elixir web apps are still web apps, so tracking the end-user experience is most valuable for businesses, as there could be sub-optimal loading patterns in the HTML, CSS, and JavaScript, and all manner of vendor JS chaos occurring on the front-end. For end-to-end performance monitoring, DataDog is pretty great, but it’s… a big tool. There’s also Sentry, but I’ve found that enabling all of Sentry’s client-side bells and whistles can have a detrimental effect on performance. There are dozens of others you can find with some light Googling.

Disclosure/shameless plug: I’m building a real-user-monitoring tool called rumdash.io focused on identifying the most costly bottlenecks affecting your users/business. You should check it out!