My Canon DSLR does not stop down to meter, it's wide open until it's actually acquiring the shot (Unless you're pushing the Depth-of-Field Preview button).
I don't think you understood. Your modern camera and modern lenses don't need to stop down to meter because the camera decides what the aperture is and can adjust the light coming in to correct the difference. On early auto-aperture SLRs where the lenses had aperture rings, the lens would relay the chosen aperture to the camera through a series of contacts, allowing it to know what the lens would stop down to.
Back in '59, they had none of that.

You figured out exposure on your own. The camera didn't know what aperture you chose; there is zero communication between the camera and the lens.
Using such a lens on a modern camera means extra work for the photographer - you need to either shoot in full manual mode, manually entering shutter speed and ISO to match the lenses aperture, or shoot in aperture priority and leave the lens stopped down all the time, which will give correct exposure. This is the same no matter what camera you mount the lens on - these old M42 screw-mount lenses can be mounted on a wide variety of DSLRs.
Because of the lack of communication, I will sometimes use the copyright field in exif to note which lens I'm shooting with, although the focal length is also recorded due to my entering it when turning the camera on to help tune the image stabilization properly. Focal length alone doesn't help with you're shooting with a variety of 55mm, 105mm, 135mm, etc M42 lenses!