@UnstableApi
    static FibonacciBackoffBuilder builderForFibonacci() {
        return new FibonacciBackoffBuilder();
    }
Returns a {@link FibonacciBackoffBuilder} that provides methods to configure
backoff delay which follows fibonacci sequence.
f(n) = f(n-1) + f(n-2) where f(0) = f(1) = {@code initialDelayMillis}
The default values are as follows:
<ul>
  <li>{@code initialDelayMillis}: {@value FibonacciBackoffBuilder#DEFAULT_INITIAL_DELAY_MILLIS}</li>
  <li>{@code maxDelayMillis}: {@value FibonacciBackoffBuilder#DEFAULT_MAX_DELAY_MILLIS}</li>
</ul>
core/src/main/java/com/linecorp/armeria/client/retry/Backoff.java