Add Omnibus config for metrics e2e tests

We are looking to add end-to-end testing for GitLab application metrics. These can be served not just by Rails, but also dedicated servers that one has to enable first.

In this MR I am adding new OmnibusConfiguration to make sure these servers are enabled prior to running the tests.

The MR that actually adds/runs these tests is here: gitlab!87906 (merged)

Part of gitlab#356982 (closed)