metalness is a grayscale map. That means black and white. Black is not metal, white is completely metal. Many types of textures use grayscale values like this.
A lot of times, you can refer to a black/white map as a mask. Because you are masking off parts, and exposing others. You could make a black/white map for your metalness, and then use that same map for something else. Maybe all the metal parts just happen to be in little nooks and crannies, so it makes sense to use the same mask for your AO. That's not a super realistic example, it's just making a point.
So you paint black on parts that are not metal. And white on parts that are metal. 99% of the time, it's just that simple!
Oh, and the color values and roughness values that you'll want to put in the metal areas -- just look up PBR values list through google and you will find official scientific measurements if you need that degree of accuracy. Marmoset has some good list, I believe, so keep an eye out for articles from them.
Replies
http://wiki.polycount.com/wiki/PBR
But if you only read one, read this one
https://marmoset.co/posts/pbr-texture-conversion/