[feature request] Need way to alert on time in certain state

[phemmer]

This is a feature request to solve an issue previously discussed on the google group: https://groups.google.com/d/topic/influxdb/1CdjyXrhbjY/discussion
I've been trying to solve this problem for months and have yet to find a solution.

The request is to be able to alert when something has been in a certain state for too long. That google group posting gives one example, but I have others, which are more relevant to other people.
For example:

Using the telegraf ntpq probe, to monitor the ntp daemon on our systems, we want to alert when the host has not had any active peer for a certain amount of time. Warn at 5 minutes, critical at 1 hour.

This logic used to determine this is: when 0 points are seen within the interval where state_prefix == '*'.

Another way to look at this is: when the time duration since the last point where state_prefix == '*' is greater than the threshold.
However this has one significant different from the above logic in that this approach needs to also handle if state_prefix == '*' was never seen, and therefore there is no starting point to calculate duration. But this logic does feel like it would have more potential use cases.

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One proposal to solve this would be a new node which takes a lambda parameter, and emits a point containing the duration for which the lambda has evaluated as true.

Usage in my scenario would then be:

data
  | state_duration(lambda: "state_prefix" != '*').unit(1m)
  | default('state_duration',-1)
  | alert()
    .warn(lambda: "state_duration" >= 5)
    .crit(lambda: "state_duration" >= 60)