False alerts for 1 hour alert threshold

[dyaffe]

Bug
We have customers with 1 hour delay materialization alerts that routinely get false alerts. Materializations can take minutes to even a half hour to produce data. Sometimes when the hour changes, we'll send an alert quickly because the current hour hasn't had data even though it's been less than an hour without data.

Expected
Customers would expect that we send alerts if no data has been seen for an hour -- not that the current hour has no data since that can happen much more quickly.