Frontend: Support multiple metric alerts

Problem to solve

We need to add the UI for allowing users to configure multiple alerts. The backend support is being addressed in https://gitlab.com/gitlab-org/gitlab-ee/issues/12258.

Intended users

Further details

Proposal

We should add this into our existing alert interface, allowing users to easily add more alerts.

One Alert	Multiple Alerts

Note: the query field is a text-entry field in this design because the original plan was that computed metrics would be allowed by the time we implemented this issue (#11879 (closed)). Since computed metrics are still behind a feature flag, this field will instead be the existing, dropdown variant.

Permissions and Security

Documentation

What does success look like, and how can we measure that?

Both the cluster health and metrics dashboards should be able to add multiple alerts

Prerequisites:

• https://gitlab.com/gitlab-org/gitlab-ee/issues/12258

MR	Status	Details
!23119 (closed)	In Dev	This is the feature flag being pushed from the backend to the frontend
!23120 (closed)	In Dev	All the new vuex related things that will handle the alert creation, deletion, updating, etc.