Android supports tagging network sockets for collecting network traffic statistics.

https://developer.android.com/reference/android/net/TrafficStats.html

applications can use this to tag a socket for collecting its stats in detail:
https://developer.android.com/reference/android/net/TrafficStats.html#tagSocket(java.net.Socket)

however apps cannot do this for Webview's traffic. This was requested a few times before (mainly gmail team). There is also an internal thread about it.

One difficulty is that webview sockets are native.