Change alert to modal when deleting a custom metric

From the discussion in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3799#note_59088109

When editing a custom Prometheus metric, pressing the “Delete” button should open a modal window for confirmation. When a user deletes a custom metric, it should return to the Prometheus integration settings page.

Current screenshot	New title	New description	New buttons
	Delete metric?	You’re about to permanently delete this metric. Once deleted, it cannot be undone or recovered.	[Cancel] [Delete metric]

Using the new design pattern from gitlab-design#187 (closed), this would look as follows: