getDelay method
Calculates the delay for a given retry attempt. Can return a Duration directly or a Future<Duration>.
Implementation
@override
Duration getDelay(int attemptNumber) {
final delayMs = baseDelay.inMilliseconds * pow(multiplier, attemptNumber - 1);
final delay = Duration(milliseconds: delayMs.round());
return delay < maxDelay ? delay : maxDelay;
}