I would expect that your chickens will stay outside the coop during the day (unless there are no other shade sources, or (if your coop is tall enough) stay close to the floor of the coop. Metal roofing, assuming its the galvanized stuff, reflects a lot of solar radiation back up. Yes, it radiates on the inside, but not significantly worse than a wooden roof (speaking as someone who laid insulation in an attic in FL in July (roof was shingles, paper, 2" XPS or EPS (the blue stuff), 3/4" ply. and plenty of heat radiating off the underside. That's been my experience, anyways.
I'm high humid environment, so all I can do is provide shade, and soil they can dig into to cool off - if you are hotter and drier, a combination of shade and misters may work for oyu to help your birds with the heat. Plus electrolytes and cool drinking water, of course.