Major Description
I think it would be a good idea to create a SubscriptionStatistics class following the same model as DestinationStatistics. There are already several counters/metrics tracked per subscription that could be moved into this new metrics class. Accessing the values programmatically would benefit because it would it is easier to just call a single gettter and receive a SubscriptionStatistics object versus having to call several getters to get individual values.
A future enhancement after this issue is complete is to expand the metrics and add new ones. For example, an inflight count is kept but it would also be nice to keep track of the total byte size of the inflight messages as well.
I will be working on a pull request for this over the next couple of days and I will push it up when it is done.