Our ReadConcern type currently exposes constants and helper methods to construct read concerns more easily. Some read concern levels have both an associated constant and method (e.g. ReadConcern::AVAILABLE and ReadConcern::available()). We should determine a single way to expose these helpers and remove redundant API. It likely makes more sense to use helper methods as some variants require input.

We should also consider applying whichever design pattern we choose here to WriteConcern, which currently has one associated constant (WriteConcern::MAJORITY).