I hypothesize the most reliably useful information is the metadata concerning each flow itself: what device is on the local end, what IP's on the other end, with what public key and cert chain in the KEX, at what time of day, and how much data is being transferred.

If you really want to see the data transferred by a specific app, inspect/modify your app's source code. If you don't have the app's source code, and you are worried about what data it transmits, what are you doing using it?