to doc

Add settings for linking to external dashboard from
metrics dashboard

This new table will be used to store the external_dashboard_url which
allows users to add a link to their external dashboards (ex Grafana)
to the Metrics dashboard.

- Move some specs into contexts
- Let get_slugs method take a parameter and return a specific slug.
- Add rescues when using Addressable::URI.

Re-use operations controller which already handles tracing settings.

This commit prepares the structure for the upcoming feature error
tracking.