static RetryRule onUnprocessed() {
        return onUnprocessed(Backoff.ofDefault());
    }
Returns a {@link RetryRule} that retries with the {@linkplain Backoff#ofDefault() default backoff}
on an {@link UnprocessedRequestException} which means that the request has not been processed by
the server. Therefore, you can safely retry the request without worrying about the idempotency of
the request.
core/src/main/java/com/linecorp/armeria/client/retry/RetryRule.java