What metrics to track usability of a analytics dashboard? [on hold]

I understand that with something like Amazon, you can track how long it takes for a person to complete their goal (ie. buy a product). But for something like an analytics dashboard, for example:

What metrics would you use to see if this dashboard is useful? Is there even such a metric? I was thinking of length of time a user spends on specific pages. Maybe widget interactions (but this wouldn't work for widgets that don't have interactions). Is the only way to see if a dashboard is useful through user interviews?

thanks.