Here is a shortlist of recommended mobile metrics you may want to track internally via your GA instance:
App Downloads & Usage
What % of your existing customer base downloads the app?
What percentage of app users make a purchase within the app?
What % of app users opt into push notifications?
Sales & Revenue
% of overall store revenue coming through the app?
The overall store revenue increase from launching an app? And in what time period?
Conversion rate on mobile app?
% increase in conversion rate from mobile web to mobile app?
% increase in repeat customer rate?
Increase in order frequency?
Increase in CLTV?
Performance increases based on using X integration?