The ability to cancel metric collections was added in HQ 4.5.1. By default the timeout value for this setting is 5s. If this timeout is exceeded, the ScheduleThread will attempt to cancel the metric collection.
As documented, the ability to cancel a collection still requires that the plugin be in an interruptible state. This means wait(), sleep() or non-blocking read(). If this is not the case, the metric is not actually cancelled and the ScheduleThread moves on.
In the past this was not a big deal since the Backfiller on the server would eventually mark the entire platform as down since a hung ScheduleThread would result in metrics not being reported. With the new design this is not the case, if any server or service Availability metric hangs, it will take until that particular metric collection times out for it to be reported as down.
This request for improvement is to have the ScheduleThread automatically send a down metric data point for any cancelled Availability metric.