Enable Prometheus by default

Enable Prometheus and various exporters by default.

issues:

• https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1957
• https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1956
• https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1955
• https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1954
• https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1953