Perhaps this need not be a 'feature' but I can't think of a good way to do this with built in options - am I missing something?

Scenario:

One or more jobs that process files in a 'source' folder. The jobs run constantly, files are touched/archived one-by-one. Is there a good way to monitor if something in the processing layer/task is askew by simply seeing if the source list is growing beyond a custom threshold? For example if the oldest file to process is more than 5 minutes old, generate an alert? Or if the number of files in the directory grows to 10 at any given time send an alert?

You can use the List files Task. This will return number of files found. In that Task you can go to the Flow tab and "On success" use an Email Notification.

As a Condition in the flow you can check if the Variable has a value higher than 10 for example.

In the Email Notification you can include Job name and number of file for example.

The Variable for the number of files found is: {TASK(Active,Result.NoFiles)}

This is the value you should use in the Condition. In the Notification you need to refer to it like this: {TASK(PrevTask,Result.NoFiles)}

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