Generally after scaling, a person may feel different for some days. Usually all dentist know that their instruments are properly sterilized, you may ask to your dentist. I would like to add here that, many times after scaling dentist may have prescribed you some mouthwash or Gum massage gel. There are chances of alter in taste because of some medications.

Normally all the instruments a dentist uses in mouth should be sterilised. This is normal to feel little different in mouth after scaling. I would suggest wait for some days if the metallic taste still persists u check with the same doctor .if it's nt painful or bleeding there is no infection