static Backoff fibonacci(long initialDelayMillis, long maxDelayMillis) {
        return new FibonacciBackoff(initialDelayMillis, maxDelayMillis);
    }
Returns a {@link Backoff} for which the backoff delay increases in line with the Fibonacci sequence
f(n) = f(n-1) + f(n-2) where f(0) = f(1) = {@code initialDelayMillis}.
core/src/main/java/com/linecorp/armeria/client/retry/Backoff.java