Learn English – Are there any common phrases in the English language that use metric units of measurement

There are a lot of expressions using second, which is metric. Of course, here the fact that the unit predates the SI system is a strong influence.

Gram is beginning to replace ounce, grain and other small units in expressions where it would once have been used, so we can find people using "gram of sense", "gram of truth" etc.

Calorie appears in a few expressions related to health, diet and exercise and as such generally referring to a "large calorie" (kcal) rather than a "small calorie" (cal) since large calories are what are conventionally used by food. This unit used to be part of the metric system, and can be considered as part of it in conventional use, though it is no longer used in the SI and so you might decide not to include it as "metric" or not.

Megahertz is sometimes used to as a metonym for internet connection speeds and computing power. "Is someone taking all the megahertz?" might be a joking complaint by someone on a shared network, and there are variations of this:

We will no doubt begin to see more such expressions as young people increasingly don't learn the obsolete units, though the history of the language, and the fondness of the US for being quaint, will likely keep them in the language for some time. After all, we still have farthing and (rarely) even sou used to mean "very small amount of money" though increasingly the people using them may have never seen a farthing, or even know what a sou is.