detect dead datapoints

I cannot express how embarrassing it is to find a datapoint has not been populating precisely when that data is needed, and this is a frequent problem.  LM needs to add a method to easily track down malfunctioning datapoints so we can find and fix these before that happens.  I have previously requested introspection checks so we can monitor longer term trends within datapoints, not just the last N faults.  This could be an extension of that -- an alert if no data in the last <time period>.  Turning on the "no data" option in a datapoint would be too noisy, but knowing a datapoint has had no data for a day would be very valuable information.

Thanks,
Mark