How can I secure NM cable to a metal stud?
Cables do need to be secured, but as you surmise, stapes and other nailed retainers wont work. There is a discussion of how to attach cables to steel studs in a brochure put out by the Steel Framing Alliance. Included in that broader pamphlet is this advice:


  NM wire is not stapled to a CFS stud because the
  staples will not penetrate the steel. Instead, wires are held
  in place with wire ties fed through punchouts in the CFS or
  with mounted head cable ties which are cinched around
  wire bundles. Mounting hole or mounted head cable ties are
  specialized ties that have a ring at one end that serves for
  fastening the tie to a CFS component with a screw. Other
  specialized wire accessories are available to re-support wires,
  like cable supports and standoffs.


Cable fasteners such as these can be used



There are also a wide variety of cable ties that can be threaded through slots in the framing or have standoffs that can be screwed to the framing such asthis type



Search steel stud cable mount and steel stud cable tie. Big box stores tend not to have them, so you probably need to seek out a specialty shop or look online.

You also can use cable spacers that work on wood or metal studs (using a screw)



It is also extremely important that you use grommets when you feed NM cables through steel studs or you risk a serious chance of shorting them out.

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