As above, metacam is a vet prescribed painkiller/anti-inflammatory. It’s not going to help your piggy relax to be groomed. Most piggies don’t like being touched/groomed but using the piggy whispering tips in the guides I linked in in order to try to encourage your piggy to trust you would be the first step. Both my piggies trust me etc, but neither like being touched or groomed. To groom my long haired piggy is a long process using herbs as treats and working gradually. I cannot fully groom him in one sitting as he doesn’t like it.
Guinea pigs have very sensitive airways so any kind of strong smelling/relaxation remedy/plug in calming type device would be bad for guinea pigs.