Add worker and slot graphs for ci-runners dashboard

Adds graphs for few new metrics that are being introduced in Runner with gitlab-org/gitlab-runner!4001 (merged).

Preview is available at:

URL	Dashboard
https://dashboards.gitlab.net/dashboard/snapshot/0b8VVczJB0JEOaBGOB8mgqzGs0gK3OE8	ci-runners: Incident Support: runner-manager