Function get_backoff_iter 

pub fn get_backoff_iter() -> Backoff 

Get a iterator of Durations which can be passed into e.g. tokio::time::sleep to observe time-based exponential backoff.

let mut backoff_durations = backoff::get_backoff_iter();
for _ in 0..10 {
    tokio::time::sleep(backoff_durations.next().unwrap()).await;
}