Not everyone gets it. Fasting puts you in nutritional ketosis as does ketogenic / very-low-carb dieting. But the metallic taste is usually a temporary and harmless keto-adaptation symptom. The body's ketone production and consumption aren't fully balanced at first for quite a while, during which some people flush out most excess ketones in urine while others breathe out more of them. As you keto-adapt (takes from a week to a few months depending on individual), ketone bodies (a byproduct of fat oxidation) are fully utilized in place of glucose by more and more tissues/cells over time, until balance is achieved.