static Backoff exponential(long initialDelayMillis, long maxDelayMillis) {
        return exponential(initialDelayMillis, maxDelayMillis, 2.0);
    }
Returns a {@link Backoff} that waits an exponentially-increasing with multiplier
2.0 amount of time between attempts.
core/src/main/java/com/linecorp/armeria/client/retry/Backoff.java