Replace metrics OmnibusConfig with Scenario class

What does this MR do and why?

Based on the work in https://gitlab.com/gitlab-org/gitlab-qa/-/merge_requests/1014

We realized that for https://gitlab.com/gitlab-org/gitlab/-/merge_requests/87906 to work, we need to expose custom ports on the Docker container running the application-under-test.

Since that requires the definition of a custom scenario, I dropped the OmnibusConfiguration and rolled it into the scenario instead.

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