Autoscaling "cores" preference should count all cores, not just loaded.

The AutoScaling "cores" preference works by counting the core names that are retrievable via the metrics API.  99% of the time, that's fine but it does not count unloaded transient cores that are also tracked by the CoreContainer, which I think should be counted as well.  Most users don't have such cores so it won't affect them.

Furthermore, instead of counting them by asking the metrics API to return each loaded core name, it should use the CONTAINER.cores prefix set of counters.