After I built my rafters I ran purlins made of 1x2 across the rafters and up the outer edges to the peak and screwed my metal directly to that. This gave me a lot of ventilation via the eaves. I also did not run my metal all the way to the peak, I left it short about 2 inches so I have a 4-5 inch opening all the way across the top of my coop, which is covered by hardware cloth and then the ridge cap.
I also put some "double bubble" insulation that I had left over from my pole barn build a couple years ago on top of my rafters before I put the purlins on. This has some insulation value and is designed in the pole barn to keep moisture from building on the metal roof and dripping down. I am assuming it will do the same in the coop and have some insulation value. If for some reason it doesn't work out I can easily cut it out from the inside with a box knife.