Lol, I'm a carpet layer, and we just call it gold metal. Lol. Or you can get silver. I wouldn't use it if your joining two carpeted rooms, get some sealing tape (they make heatless now for small jobs) and then it looks real professional if done right. But if your going from carpet to tile use z-bar and tuck it under. If going to linoleum or other sheet goods then gold or silver 'bang down' as we call it works real good. Any store that sells it is going to know what your talking about. But a 'threshold' is the politically correct terminology for any doorway floor trim. Originally designed to hold the 'thresh' they used to use on floors inside the home or room.
Te metal that goes on top of the carpet in order to separate the rooms are called a threshold strips. It joins the carpet nicely at the doorways. Other special types of metals are used in joining the carpet to hard floors. There are two types of these Threshold strips that for joining the carpets and that one for finishing off.