[Garlef]

The correct term would be "composition in an enriched category". The morphism objects in this context are usually not called arrows.

I think the other poster meant "elements in the set of morphisms" when they said "arrows".

The difference is the following: Metric spaces "are" categories enriched over the real numbers, ordinary categories are categories enriched over the category of sets. So in one case the morphism objects are sets while in the other case they are real numbers. "Arrows" then refers to something internal to the morphism object. The distance (a real number) between two points in a metric space does not have any internal structure.