The metal in your pads is galled metal that the pads roll up off the brake track then break off when they become embedded in the pad faces.
With those bits stuck in the pad faces you now most certainly have big ugly gouge marks in your braking track on the rims. Those grooves are where the metal in the pads came from.
It seems like the bigger the rider the easier the cheap Tektro and Shimano pads pick up and gall like this. Likely because us heavier riders have to use more lever pressure to stop. As a result at the first sign of this I toss those style pads and put on Koolstop or other brands that I know won't gall the metal in the same way.
And best of all the "fingernails on the chalk board" sound that the bits of metal in the pads make will be gone with the pads. Contrary to some folks the pads are not supposed to sound like rubbing two slabs of steel together with gravel between them....
