End alignment evaluation in alert queries can be incorrect if the offset is zero

[cwbriones]

When we check if an alert is in the firing state, we validate that

1. there is data and
2. the data isn't stale.

(2) essentially means that the timestamp of the most recent datapoint should be less than a period away from the end of the query window.

However, when a query specifies end-alignment, the most recent timestamp is always now - aggregationPeriod which is exactly one period away if the query offset is set to zero. This means with this configuration we'll always mark an end-aligned alert as OK even when it should be firing.

In practice, queryOffset is always >=1 so this problem doesn't occur, but it still may be worth fixing if there scenarios where we would want to set the offset to zero.

[cwbriones]

References

Where we check the alert is firing: https://github.com/ArpNetworking/metrics-portal/blob/master/app/com/arpnetworking/metrics/portal/alerts/scheduling/AlertExecutionContext.java#L236