My public extension has over 100 installs and about 5/10 active streamers on a daily basis, but it is for a niche game (Elite Dangerous) which very very rarely gets a “big streamer” so on a daily bases maybe 2k viewers see my extension.
My private extensions have 1 streamer installed each, and north of 20k viewers seeing/using/interacting daily (as a ball park figure I’ve not done the math, just remembering average viewer counts ish). I’ve not actually run metrics here.
At the end of the day, metrics for individual extensions won’t make much sense since different extensions do different things and different extensions are not gonna be that forthcoming about interaction metrics, (that RPG game is gonna have HUGE interaction scores, but a POE Loot extension will have way way less since it’s just on/off click item for more details etc).
Twitch hasn’t provided any random metrics (or I don’t recall see anything actually worth using for a business case), and this is just a guestimate for my three extensions.
If you are building something for a game, then it’s a matter of determine how long a streamer is gonna play that game and whether that extension adds anything to the experience of the game.
(Finally whilst Twitch staff do float on the forums it’s mostly third party guys like me round here kicks back and puts feet on the coffee table)