Making instance groups more useful

Being able to set (and keep) thresholds on instance groups from the DS and/or group level would allow us to consolidate our “network interfaces alerting” and “network interfaces informational” datasources (which are clones except that one has thresholds and the other doesn’t) into a single DS.

Being able to use instance groups in alert rules would help because we’d have the interfaces sorted manually into either “alerting” or “informational” groups and only route alerts on the alerting ones.