If you install it once and then remove it, you only do a single download. If you install it and keep it installed for several app updates, you download it for every update. If a lot of people download it at the same time due to an app update, you'll see a huge download spike immediately after a release.

It's a very rough metric, but it could be added without adding any kind of additional telemetry to the client, and it would give at least a small hint of what's happening.