How to add a third party prometheus data source?

I I’ve run the grafana:master-ubuntu image and I’m trying to use it to monitor a Triton server’s metrics. I have added the data source by selecting a Prometheus as the source and put in the URL (which i’ve confirmed both in my browser and curl from within the container) but i get HTTP Error Bad Request with Access set to Server. I then tried Access set to browser and got Unknown error during query transaction. Please check JS console logs. which is a CORs error, but i did notice Grafana has added /api/v1/query?query=1%2B1&time=1594357629.928 to the URL? which is causing the error I presume as Triton is expecting only /metrics.

So how do i stop Grafana adding the extra junk to the URL?

Ahh, so i guess i have to add the source to Prometheus and then use Grafana to read them from there!