It means your dog's internal pipes are rusting. A very serious condition. You can prevent it by regularly flushing the pipes.
How often do you suck your dog?
In all seriousness metal smell and taste is normal. My dog has a metallic taste when he has not been sucked for a few days.
This is just the smell and taste of dog dick. You shouldn't try to prevent it as his dick and sheath is self lubricating, but I'm sure you'll learn to like it