It seems that blender still does not support Metalness baking in 2.8. This is essential to asset designers, will there be support for full PBR map baking anytime soon? and are there any Free programs or add ons that will allow me to bake out a full set of PBR textures specifically the Metalness map for a mesh?
Partially true, it is somehow not very straight forward or obvious, but here is a work around:
remove - disable all of your lights from your scene (as you do for the rest of your bakes if you dont want to affect the baked textures)
Bake Glossy (only Direct Enabled)
As has been mentioned in the comments it seems that there is no color bleeding on the faces, but instead if any face is occluded will be affected by shadows (which is not wanted of course) as it happens on my example (see bellow)
So far I didnt find a proper way to get rid of it, in fact what I had to do was to remove the plane in order to get the appropriate result, which is a bad thing.
Nevertheless, this should do the job for now ;)
Just in case some else finds this like me. The best method is to plug your metallic input into roughness and then just generate it as roughness but save it into a metallic texture. Then reconnect back to where it should be. Simple and effective without any shading (and roughness generates much faster than diffuse).