You asked about the rim. but showed the pads ..
How do people measure the amount of brake metal left on a rim?
to be accurate a thickness measurement of the new rim would need to have been taken , where the brake track Is ..
then a more recent measurement again , to actually have the Data to Judge/quantify rim wear.
the bead hook gets in the way , so the caliper would be notched , or a piece that fills in behind, inside the rim is kept and used again .
As the rim wears , so as to maintain the record of the same test conditions.