Thanks. I am not expert. If it was my bag , I would remove the old leather, and cut same color very thin leather strips, same width or 0.5 mm wider than the metal cap(0.25 mm at each side), long enough to cover up the whole circle ( make sure the ends meet with no gap), then glue the leather onto the metal cap.
My favourite glue is E 6000 fabric fuse, it seems week at beginning, requires clamp & un disturbed for 72 hrs. Feibing’s leather cement will work too, just follow the label instructions. Good luck with the repair.