How to measure the width of a door with an extra ring?
This looks like a Euro Cylinder Lock. You can find resources on the Internet explaining how to measure and replace them.

The silver ring is an escutcheon. It is a trim piece that covers the rough hole in the door where the cylinder goes through. It is likely to be fixed to the white part of the door. You might be able to remove it when the cylinder is out, but it is probably best left in place.

Therefore the measurement for the right side is 30 mm. I can't see what is happening on the left side so I won't comment on that. You can check the measurements more easily by temporarily removing the cylinder.

Try to get a cylinder size that protrudes as little as possible and also includes anti snap features.