Add custom metric e2e test

What does this MR do?

This MR covers an existing gap for an EE, Gitlab Ultimate feature for the APM group (Monitor stage) tracked with the following test case: gitlab-org/quality/testcases#259 (closed)

The MR includes the test and the refactoring necessary as the first layer of structure for upcoming metrics.rb tests (introduced the /operations/metrics/show.rb). This is also the first iteration of the CRUD in metrics as the test only adds a new custom metric.

Availability and Testing

This test introduces 10 additional minutes in the test run. This is an alarming situation that will be taken into account when architecting the next Monitor related tests, since the setup of a ready-made monitoring project is costly.