Spike: Support alerts on custom defined metrics/dashboards

Currently when trying to add alerts to custom metrics on custom defined dashboards, while the tooltip that shows the alert was created and what kind of alert was created, there's a bunch of undefined fields in the text, e.g.

We should investigate to better understand what are the implementation issues needed to support alerts on custom metrics/dashboards as well

Testing and Availability

There should be tests to cover all levels of test pyramid: unit, integration and e2e. E2e test is scheduled for development.

Discussion Summary

1. The backend should respond with an error status (and error message) when it cannot create an alert. Currently, it responds with http status code 204
   • Follow up issue #213617 (closed)
2. If possible, the frontend should not show the alerts option in the menu for metrics which cannot have alerts. The dashboard API response should contain some indication if a metric supports alerts or not.
   • This was fixed in #204796 (closed)
3. Support for alerts on custom metrics
   • area-charts/line-graphs
     • #28241 (closed) (Scheduled for 12.10)
   • Other charts
     • #28242 (closed) (Scheduled for 13.1)