Civilization has had a habitable planet for the entirity of its run. When food and water are scarce, what good will metal do?

Yeah, when food and water are scarce, rearing sheep will be hard too.

Also what are you going to do when your tiny gold coin is worth thousands of dollars? Shave off tiny flakes of it and trade it for eggs?

You could write on a piece of paper that says you promise to give the bearer of that paper gold of the equivalent value..