Admob has an impressions metric but I am not sure if it's per ad display, or the unique display per user. If the same ad is shown 100 times to the same user on their app, does it count as 1 or 100?
As long as you're not reusing the same ad, yes. Each time you make the call to load a new ad, you have an opportunity to record an impression. If the AdMob server chooses to send someone the same ad for cat sweaters twice in one day, it doesn't matter. You're still credited with two impressions because you made two requests and showed two instances of the ad.