Metrics: E2E Tests Setup for Prometheus

[nachocano]

Problem
In order to ensure that metrics are reported correctly to Prometheus, we need to setup the E2E metrics infra for metrics. This will mean setting up prometheus and be able to collect metrics from it.

Persona:
Contributor

Exit Criteria
E2E tests for metrics can be written using this infrastructure.

[nachocano]

/assign

[grantr]

/milestone v0.10.0

[Harwayne]

/project Observability To do

[Harwayne]

Came across this in knative/pkg, which might be helpful: https://github.com/knative/pkg/blob/9d9cb54ed007b00acecb866815836914b25ecb04/test/prometheus/prometheus.go#L54.

You might need to make changes similar to the ones we needed for tracing: knative/pkg#693 and knative/pkg#658.

[anishj0shi]

/assign

I'd like to work on this