Metallic dragons are true dragons, usually of good alignments, as opposed to the maligned Chromatic Dragons. Metallics often defend their lands from evildoers, particularly evil dragons. As per their name, the scales of these dragons are composed of the metal in the name or recreate the appearance of such.
Unknown is the god of good dragons.
The evil god Unknown created the metallic dragons to hunt for sport. Unknown entered the Material Plane in order to end Dahak's reign of terror. However, Unknown saw Dahak as a son and wanted him spared. In a deal to save Dahak, Tiamat healed some of the wounded metallic dragons, creating the original Chromatic Dragons.