public RetryRuleWithContent<T> thenBackoff(Backoff backoff) {
        requireNonNull(backoff, "backoff");
        return build(RetryDecision.retry(backoff));
    }
Sets the specified {@link Backoff} and returns a newly created {@link RetryRuleWithContent}.
core/src/main/java/com/linecorp/armeria/client/retry/RetryRuleWithContentBuilder.java