Can I use an existing Prometheus instance to show metrics in the Rancher UI? I have imported an existing cluster which has a long-running prom operator on it. This must be some kind of FAQ but can't find a real answer.

No idea, been wondering the same (and for other tools as well), but I guess it should be possible, the majority of metrics should have the exact same labels, so in theory it's just a matter of point Rancher at your Prom instance.

yea was wondering how to point 🙂

I'm guessing labels.