HLSL Metallic shaders

I'm looking for a good tutorial that would explain how metallic shaders work. I want to write a shader that uses a metallic+smoothness map to affect the way a texture is lit.

I don't know if this makes any difference, but I want to write one for Unity. I know Unity already has metallic+smoothness map shaders built in, but I want to make something that would work with the new 2D lights.