Export storage class information

[ribbybibby]

See #29 which introduced this idea. It would be useful to be able to reason about the metrics based on storage class.

My thinking is that we could add a storage_class label to metrics where it makes sense. This would allow you to do useful things like compare the number of objects that have been transitioned to glacier vs those in standard.

This would be a breaking change as it would change the output of a number of queries when there are more than one storage classes. I don't mind cutting a major version for this but it might make sense to think about any other breaking changes I want to make that could be batched into the release too.