Is my microwave still safe to use after accidentally heating metal in it?
The spark was a voltage jumping over a gap somewhere. This may occur again, but so long as the magnetron survives, you should be okay.  The problem with sparks is they can leave a carbon trail deep inside which is conductive and may actually make the next spark occur at a slightly lower voltage. If this happens enough, you will eventually (or shortly) damage the unit and make it unusable.  

I actually work as a product safety consultant and I can say, so long as you have an approved product (CSA, UL, ETL, or TUV) this sort of fault was actually tested. You should be a safe enough and not start a fire.