Ask HN: What do you use to monitor website performance from client perspective?

Also known as "Real User Monitoring". There are a lot of SaaSes doing some form of it, but I know none avoiding third-party JS or requests and most don't support First (Contentful) Paint. What's your experience? What do you use? If you hacked something together in-house, what stack did you use and how do you build reports on top of it?