field_descriptor_names is a list of the protobuf name (not fullname) of the field which contains the interface as google.protobuf.Any (the interface is the same, but it can be in multiple fields of the same proto message)
fullname is the protobuf fullname of the type containing the interface
InterfaceAcceptingMessageDescriptor describes a protobuf message which contains an interface represented as a google.protobuf.Any