Function check_bucket_at 

fn check_bucket_at<K, Q>(
    table: &Mutex<HashMap<K, (Instant, f64)>>,
    key: &Q,
    make_key: impl FnOnce() -> K,
    rate: f64,
    burst: f64,
    now: Instant,
    cap: usize,
) -> Result
where
    K: Borrow<Q> + Clone + Eq + Hash,
    Q: Eq + Hash + ?Sized,