static RetryRule onException(Class<? extends Throwable> exception, Backoff backoff) {
        return builder().onException(exception).thenBackoff(backoff);
    }
Returns a newly created a {@link RetryRule} that will retry with the specified {@link Backoff} if an
{@link Exception} is raised and that is an instance of the specified {@code exception}.
core/src/main/java/com/linecorp/armeria/client/retry/RetryRule.java