static RetryRule onUnprocessed(Backoff backoff) {
        return builder().onUnprocessed().thenBackoff(backoff);
    }
Returns a {@link RetryRule} that retries with the specified {@link 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