The Doctrine\ODM\MongoDB\Proxy\Resolver\ClassNameResolver interface has been
deprecated in favor of the Doctrine\Persistence\Mapping\ProxyClassNameResolver
interface.
The getClassNameResolver method in DocumentManager is deprecated and should
not be used. To retrieve the mapped class name for any object or class string,
fetch metadata for the class and read the class using $metadata->getName().
The metadata layer is aware of these proxy namespace changes and how to resolve
them, so users should always go through the metadata layer to retrieve mapped
class names.