Exponential
@Serializable
Exponential: delay = initialDelay × 2^attempt, capped at MAX_BACKOFF. delayFor(0) = initialDelay, delayFor(1) = 2 × initialDelay, delayFor(2) = 4 × initialDelay, …
attempt is 0-indexed (see Linear).
data class Exponential(val initialDelay: Duration = 30.seconds, val maxAttempts: Int = DEFAULT_MAX_ATTEMPTS) : BackoffPolicy