I agree it's not great, but I spent a while trying to come up with something, and I couldn't come up with much better. `metadata` was my first shot, but that could mean a few other things in some contexts (i.e., we already use it in `NDData` to mean "stuff you might need to know about how/when this image was taken or similar").  

Anyone else have ideas?

Even if there aren't others, I agree that `frame_spec_attrs` is an improvement, so at a minimum it should change to that.  I won't do it in the current set of changes, though, so as not to blow away this comment in the PR