Programmatically, I understand how to manage a single users subscription, but have not found a way to retrieve a holistic picture of In-app Subscriptions.
The closet thing I've found in the In-app Subscription documentation deals with tracking subscriptions to order numbers:
To help you track transactions relating to a given subscription, Google Wallet provides a base Merchant Order Number for all recurrences of the subscription and denotes each recurring transaction by appending an integer.
Using this mechanism in a roundabout way one can determine current subscription orders, but this doesn't give a clear picture of subscribers. How do you enumerate the number of users who are currently signed up for an In-app Subscription?
The Finance overview for an app in the Google Play Store Developer Console provides a summary of app revenue. In addition to App Sales the Fiance screen details a summary for Subscriptions which breaks down revenue by: day, last 7 days, last 30 days and overall.