if low level discovery rule does not discover some entity anymore, we show a warning on the frontend, but there is no way to alert on this.

for example, we can monitor filesystems for used space, but not for availability.

maybe that could be another internal event type ?

"New LLD entities discovered" / "Previously discovered LLD entities lost"

the last would be a bit bad with period of 0 - could we have some way to notify user about which entities were lost ?