Function sprint 

pub fn sprint(
    sprint: &Sprint,
    counts: Option<(u64, u64)>,
    today: &str,
    ctx: &Context,
) -> String

One sprint, and how far through it is.

Two ratios, because a sprint that is four days from its end with half its issues open is a different situation from one that has just started, and a list of dates makes the reader do that arithmetic themselves.

counts is (resolved, total), and absent when the issues were not asked for: two counts are two requests, and a caller who only wanted the dates should not pay for them.