Add GitLab Runner contacted_at metric to exporter

Problem to solve

Monitor and alert on GitLab Runners who have not contacted the GitLab server recently.

Further details

Currently we periodically check runners' contacted_at through the GitLab API. Then we send this to our alerting system. It'd be a lot nicer if this was exported as a metric and Prometheus/AlertManager can alert on it based on thresholds.

Proposal

Add the GitLab Runner contacted_at metric to the GitLab Prometheus exporter.