SLI alerts for library metrics (from additional_metrics.yml)

We ran into an issue when building the MVC of SLI alerts, because our out of the box metrics are loaded from a YML file, which does not get into the database. This means that we have no unique identifiers, and other issues as noted here: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5163#note_88185734

We need to do the following:

1. Extend the metric data model in the database to support all the features of the YML file
2. Import the YAML file into the DB on startup (or some other method)
3. Utilize the DB as the source for all metrics, as opposed to reading in the existing YAML file.
4. Consider how we would update this going forward (db migration?)

The UX remains unchanged, just now available as well for default metrics.