It would be great if we could save some render stats in the metadata of our files like when you take an actual photo.
Just a thought 
Ah so its down to max handling the saving of images.
No problem
yup, not possible 
But still this is possible for the CXR format, and is actually done right now. I am not sure what exact data is saved, but I remember some improvements were planned, so if there is a need for some specific data, it could be probably added.