As the previous poster replied, it's related to https://en.wikipedia.org/wiki/Ketosis
I don't know if you can really get rid of the taste, I'd be curious to know of course; but you kind of get used to it really.
I personally don't mind taking fruit juice, as it does give a small energy boost, especially in the morning.
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.
I personally don't mind taking fruit juice, as it does give a small energy boost, especially in the morning.