The glass lens on my fog light got hit by a rock and has broken. Its just the lens, not the actual light. (it still functions)

Has anyone had this happen to them?
Is it covered by warranty?
Would it be simple enough to replace myself?

doubtful that it's covered under warranty since it's an accident and not a defect.

Assuming it's similar to the earlier fogs except for size, the lens won't be available separately. You'll have to buy a complete replacement lamp assembly.
It will definitely not be covered under warranty, don't even ask.
To prevent this from happening again in the future, get some Xpel or Stongard material and apply it to the fog light lenses. It will prevent breakage from normal road-debris impacts.