What is this metal inside my wall?
Judging by the thickness and location of the metal you detected it seems like you might have metal drywall corner bead, which is typically installed on the edges of drywall to reinforce the area and prevent chipping.
Assuming that is what you found, the corner bead shouldn't interfere with the installation of shelves. That doesn't mean there isn't anything else behind the drywall that the stud detector isn't picking up, so proceed with care.