We currently track APK size, but don't report how much disk is being used at runtime. We should add UMA for this!

It would essentially do a recursive ls on the app's data directory and report the sum.

Alternatively, we could report the disk usage any time someone visits Chrome's disk usage activity (which itself calculates cache vs non-cache numbers).

In order to make this data actionable, we'd want to break it down into buckets that make sense. E.g. cache vs non-cache makes sense, but are their sub-buckets that would be useful here as well?