While holding down the Command key, click the "Metal" layer to load the selection (Click on the thumbnail of the layer, not on the layer name). With any selection tool selected, move the selection 4 pixels right. You can do it by pressing the Right Arrow on the keyboard 4 times.
Go to Select > Inverse. While holding down the Command + Alt + Shift keys, click on the "Metal" layer. Alt + Shift combination is for intersecting the already selected area and the new selection that we're going to make. The new selection is the metal part because we're holding down the Command key as well. So the metal part and our current selection will be intersected. Using this selection make a thin line on the metal part selected. Use these lines to make the bumps.