Query to detect missing time series

[Peter S]

Hi,

I'm trying to come up with a query to alert on time series that were on a metric but are now missing, such as in detecting node become unresponsive and time series in up{} become absent. 

It is easy to enumerate the known label values to achieve this, like absent(up{instance="abc"}). However, I wonder if there is a good way without having to enumerate label values to achieve this. 

Thanks,

Peter