XXX_ types are used by the Protobuf library to store unknown fields. When you decode a proto, there may be additional fields in the serialized data that the library doesn't know what to do with. This can happen, for instance, when the reader and writer of the data are using different copies of the proto file. This is a feature to help with backwards compatibility between clients and serves built at different times.
As for what you should do with these? I would just leave them alone, and don't reference them. You don't need to set them, and you don't need to read them. The Go protobuf library will handle them for you.