POSITION THE RAIN GUTTER RIGHT
The rain gutter should be installed a bit lower down so it is not directly in the path of the sliding snow, and the metal roof should hang almost halfway over the gutter. You will have to adjust all of this according to how regular rainfall flows off the roof -- you don't want the water to overshoot the rain gutter. So study your roof on a day with heavy rain and see just how far you can drop the rain gutter and still collect the rain. In addition you could put a "gutter protector" over the gutter, designed to stop leaves from getting into the gutter. These would grab ice, but snow would flow over the top.
You can also get snow dams that are glued or screwed to the roof higher up, to limit the constant flow of snow, without limiting the movement of liquid water. The one we showed is called Polar Blox made by Vicwest.
One very interesting product for snow country is called Gutter Helmet. It is a specially shaped sheet metal cover for the rain gutters that draws the water around and into the gutter but throws leaves, snow, and even ice off the edge over the gutter. Leaf Guard uses the same concept but is built right into the rain gutter. Follow this link for more details on Gutter Shields.
LEAVE THE RAIN GUTTER OFF?
One of my radio listeners who is renewing a metal roof was told by the contractor that he should not put on a rain gutter, it just causes problems. Yes, as you can see above, rain gutters, especially on slippery metal roofs can create problems, especially if they are not installed solidly and in the proper position. But most roofing problems are caused by heat escaping from the house melting the snow pack from below and/or poor attic ventilation under that roof. The absence of a rain gutter can lead to foundation problems because of a lack of water run-off control. So I strongly support rain gutters and I prefer to shift the burden to the roofer to have him deal with the entire roof system (house air sealing at the ceiling below the attic, attic insulation, attic ventilation, proper clearance between the outside walls and the underside of the roof) rather than let him say that his job is only the roof covering. Even the best roof covering cannot work well if the rest is not correct. A properly built roof will not have problems with properly installed rain gutters and now both the top and the bottom of your house will be protected from water penetration.