Ask HN: Open-source NewRelic / DataDog APM alternative?

I'm curious if there is a good open source alternative out there for NewRelic or DataDog's APM service. I see lots of options for log aggregation or server monitoring, but haven't found anything with application integration so you can trace transactions.

Moskito is probably the closest thing that I've seen. We've done some custom wrappers using JMX and other things but with sidecars instead of directly integrated to Java.