Function exponential_backoff 

pub fn exponential_backoff(
    initial_delay: Duration,
    multiplier: u64,
    max_delay: Duration,
) -> impl Iterator<Item = Duration> + Clone

Creates a backoff starting at initial_delay, multiplying subsequent delays by multiplier, and capping every delay at max_delay.

This wraps [ExponentialBackoff], whose from_millis argument is the mathematical base rather than the initial delay.