Codahale/metrics will send a value of "Infinity" as a value, or at
least it can be made to do so for a ratio value.

This is represented in a repr as ", inf)" but that can't exactly be
evaluated - e.g. you can pass that to the constructor for float, ala
float("inf"), but by itself it raises a NameError.

Setting inf=-1 allows the evaluator to move on.