Add measurements to get better visibility into reported late & missed tab alarms

[lmorchard]

We've had reports of missed alarms for waking tabs. We've not been able to reproduce the condition so far.

Could be useful to add measurements to try to get a better sense. Maybe measure how late a tab wakes up past its scheduled time. Not sure how to measure tab alarms we miss altogether, since that would probably rely on the same code that missed the alarm in the first place.

Not sure what the action from the measurements would be, other than maybe getting a sense of how common the issue might be?