Here's one that works.
(2,1): 1
(3,1): 2
(3,2): 3
(4,1): 4
(4,2): 5
(4,3): 6
(5,1): 7
(5,2): 8
(5,3): 9
(5,4): 10
The generalization to an n-vertex complete graph should be clear. The proof of correctness is inductive. For n = 0, it's obvious. For higher n, the inductive hypothesis is equivalent to the proposition that every violation of the triangle inequality involves vertex n. The edges involving vertex n are longer than the others, so n is not the transit vertex of a violation. Thus every hypothetical violation (up to symmetry) looks like n -> v -> w. There exists some constant c such that n -> v has length c + v and n -> w has length c + w. Hence, if v -> w is a violation, then it has length less than w - v, which, by inspection, is impossible.