Enable adding custom metrics for all gl-managed prometheus scopes

Problem to solve

Custom metrics are not enabled when setting up gl-managed prometheus at the instance level (potentially also at the group level).

See discussion at https://gitlab.com/gitlab-org/gitlab-ee/issues/13564#note_204707530 for more context.

Intended users

Further details

Proposal

Enable custom metrics for all gl-managed prometheus scopes.

Permissions and Security

Documentation

Testing

What does success look like, and how can we measure that?

What is the type of buyer?

Links / references