Follow-up from "Support user-defined links in metrics dashboard"

The following discussion from !33324 (merged) should be addressed:

• @rpereira2 started a discussion:

  suggestion (non-blocking): We should add this new element to the spec/fixtures/lib/gitlab/metrics/dashboard/schemas/dashboard.json and spec/fixtures/lib/gitlab/metrics/dashboard/sample_dashboard.yml files as well.

  We can do that in a separate MR though.