static Backoff exponential(long initialDelayMillis, long maxDelayMillis, double multiplier) {
        return new ExponentialBackoff(initialDelayMillis, maxDelayMillis, multiplier);
    }
Returns a {@link Backoff} that waits an exponentially-increasing amount of time between attempts.
core/src/main/java/com/linecorp/armeria/client/retry/Backoff.java