The metal key is gained during and after the Tourist Trap quest. During the quest, it is first obtained by killing the Mercenary Captain in order to gain access to the main gate of the Desert Mining Camp.
This key can be placed on the Steel key ring.
If lost another key can be retrieved by killing the Mercenary Captain (located outside the camp) again or, if the quest is completed, by searching one of the tables in the building located in the north part of the mining camp.
For an exhaustive list of all known sources for this item, see here