private static CircuitBreakerRule rule() {
        return CircuitBreakerRule.builder()
                                 .onException().thenFailure()
                                 .orElse(CircuitBreakerRule.builder().thenSuccess());
    }
Returns a {@link CircuitBreakerRule} which returns {@link CircuitBreakerDecision#success()}
when there's no {@link Exception} raised.
core/src/test/java/com/linecorp/armeria/client/circuitbreaker/CircuitBreakerClientTest.java