Enable Prometheus metrics for all GitLab QA tests

We should ensure Prometheus metrics is turned on for all GitLab QA tests, to ensure it works across all test cases.

In 10.2, we found an issue where the increased number of metrics caused crashing quite quickly. We had to release an emergency patch to address this.

By enabling it for all Prometheus metrics, we will also exercise a wide array of the instrumentation as well. Often times the metrics will not begin to be generated without someone hitting the route first.