Has anyone used an "hourly active users" metric to measure mobile app engagement?

I track HAU w.r.t Videos consumption in my product.

I use GA to plot trend lines of today with past day & see how things move. Graph line is then manipulated using pushes or content change to see the impact.

Check users & events w.r.t time of the day.

Regardless of why I was going to interview a customer, I had a checklist of what I would do before getting contacting the customer directly. (This is for a B2B customer and enterprise class software. Your needs may vary.)

1). I would contact customer service and get contact records for the last 6 months and see if anything unusual has been happening in our relationship.

2). I would contact our accounting department and check to see if they have had any near term financial issues.

3). I would contact their sales rep if they had one and make sure that my visit would not cause friction with any activities they are involved in and to get additional background information.

4). Once I had that completed I would weigh through the issues if there were any. Only then I would contact the customer and ask for an interview. (At times an interview could help solve some of the above issues, at other times they would not, so another customer was selected.

Depending on why the customer was being interviewed would drive the next set of processes.