make a keg of kool aid and carbonate it and see if the taste sucks.
I would use lemonade to avoid staining your lines. Not sure if it would, but just to be safe.
I guess it would be possible when they did the Hydrostatic Pressure Test, they did not purge all of the liquid from the tank causing internal corrosion. I would take it off your beer keg and bleed the c02 till it all comes out of solution. I’d take the tank back and have them swap it out. It’s a possibility your beer may be ruined but, why risk future batches. Try the new tank on your current problem batch and see if the metallic taste dissipates.
Another thought might be you had a strong concentration of iodophor and didn’t rinse well enough? Just trying to give you some ideas.