A datasource to alert if SNMP is not working

There is a way to do this, but it is not well-documented and there is no UI exposure for "No Data" alerts, you have to dig around the module sources to find them (because it is very hard to put an indicator in the alert tuning thresholds I guess). 

We have standard alerts on 2 datapoints that have No Data alerts and no other alerts.  The first is for "Host Uptime" -> SNMP_HostUptime_Singleton -> Uptime and the second is for Uptime- -> * -> UpTime.  If a host stops responding to SNMP, those will trigger.  We keep them near the end of our alert policy to generally report to our team across all clients.