Spectral highlights on metal are difficult. I'm not an expert. A white reflecting card above may even out the lighting some; but with mirror highlights evening out the lighting often doesn't help much. Could try a polarizer on the lens, rotating it to adjust the amount of highlight. Sometimes they even spray the metal with a matt paint to reduce glare for product shots. As I say, I'm really not an expert. Some of the other folks here have been getting some good metal shots lately. Maybe they can help more.