BsonClassMap __classMaps is static mutable, with popsicle immutablity.

This makes:

• Testing class maps in isolation is impossible.
• Prevents parallel testing because setup is not thread-safe and (negative productivity effect)
• Doesn't allow runtime service resolution of things like jsonserializers.
• Problematic when using two or more seperate mongo databases in same process space (they end up sharing ClassMaps).

Instead, perhaps, add a BsonClassMaps custom collection, move the static methods to there as non-static and allow it to be injected into a MongoClient (vis MongoClientSettings).

services.AddBsonClassMaps(map => map.RegisterClassMap<>...);