Previously, we only enabled metrics when the playbook was installing
Prometheus (as indicated by `prometheus_enabled`).

We are exposing metrics when `matrix_metrics_exposure_enabled` is
toggled to `true` though, but people need to toggle various
`_metrics_enabled` variables to make services actually serve metrics.
No more. If `matrix_metrics_exposure_enabled` is `true`, we'll
automatically enable metrics for all services.