The "simplest" self-hosted tools are probably Prometheus (https://prometheus.io) and Jaeger (https://jaegertracing.io), but they're big distributed systems. Running them may or may not be worth the cost, though you might be using something like Prometheus already.

I really would love to see a simpler set of tools that works better for small / medium companies to self-host if they really can't send data to a vendor for some reason.