REGISTER_DOCUMENT_SOURCE_WITH_FEATURE_FLAG checks the value of the feature flag/FCV at startup and registers the document source appropriately. However, if you downgrade the FCV from an FCV where the feature flag is on by default to an FCV where the feature flag is off by default, the document source should error even though the document source will remain registered.

We should consider changing the registration macro to wrap the "parse" function provided with a feature flag check, so that the feature flag is always pessimistically checked if registered with this macro.