Connecting to Grafana: error reading Prometheus

[deployn]

Hey DenBeke,
thank you for creating mailbear.

I have a question regarding the Grafana setup. I proxied a domain to port 9090. Now, when I enter the domain/metrics, I get something like this:

I tried to add this URL as a data source in Grafana.

Grafana returns the error Error reading Prometheus: client_error: client error: 404

What am I doing wrong?