Add an integration test for ingress thru Traefik

[sinapah]

Enhancement Proposal

In Mimir, we test ingress through Traefik. In Prometheus, however, we don't have an integration test that relates Traefik to Prometheus and ensures that a metric generating charm can successfully send remote-write telemetry to Prometheus.

We should write an integration test performing the following:

• Relate Prometheus to Traefik and relate something like Avalanche to a collector like Opentelemetry Collector. After relating Otel to Prometheus, ensure telemetry from Avalanche is successfully remotely written to Prometheus at the "ingressed" endpoint.