Draft: Enable monitoring Review Apps via Prometheus and Grafana

What does this MR do?

Enables Prometheus as initially proposed in !52604 (closed)

This is done to provide more metrics that might help troubleshoot performance, stability, and availability problems that we encounter frequently when running QA tests on Review Apps.

See #334342 (comment 616970725)

Does this MR meet the acceptance criteria?

Conformity

• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.

Availability and Testing

• I have tested that this change does not cause problems with existing deployments