My old house was somewhat similar...perhaps worse in some ways, 12/12 pitch metal roof over the great room, with the chase/chimney at the outside center, (daylight basement made it 3 stories up, added interest), the "rear" portion of the house was shallower, only 4/12, also all metal. My approach was to get on the 4/12 part, straddle the peak and "shimmy" my way to the chase, on my butt. I.d to yours the same way, go up from the garage side if you have a long enough ladder (ground looks flatter) and work across. What's your pitch? You can get along fine on metal with a 4/12, passibly on 5/12 (as long as its dry and dust free), anything steeper, hire it out!
BTW, "pucker factor" will hold you in place on the ridge!
Bri