Add 'Idle Efficiency' graph to ci-runners autoscaling dashboard

Idle Efficiency graph is an estimate of how much efficient is our idle instances amount. If there are way more instances in idle than any others, it means we're probably over-provisioning too much (that's not true in all cases, especially when there is no constant number of incoming jobs in the queue for a given shard).

The graph should be analyzed with others containing raw numbers, but itself is a good overview of what's the general state of the shard when it goes for costs of maintaining idle instances.

Dashboard preview:

URL	Dashboard title
https://dashboards.gitlab.net/dashboard/snapshot/aXY6cR4kaoXNxYsMAAWsbZf8Z9c3cH0h	ci-runners: Incident Support: autoscaling