> But where's the difference between not being able to pull vs. not receiving metrics for a period of time?

Let's say I have a service that's only used once per day, at a random point in the day. I need to know if the service is available all the time, even if it's not used. With pull, I'll get back an 'OK' e.g. every minute. You could say that we should implement heartbeats, but again, then this service needs to be configurable to pick up changes in the monitoring infrastructure, then I need to have a separate check for the presence of heartbeats and the values of the samples..