@UnstableApi
    static FixedBackoffBuilder builderForFixed() {
        return new FixedBackoffBuilder();
    }
Returns a {@link FixedBackoffBuilder} that provides methods to configure
backoff delay which is a fixed value. The default values are as follows:
<ul>
  <li>{@code delayMillis}: {@value FixedBackoffBuilder#DEFAULT_DELAY_MILLIS}</li>
</ul>
core/src/main/java/com/linecorp/armeria/client/retry/Backoff.java