private static CircuitBreakerRuleWithContent<RpcResponse> rule() {
        return CircuitBreakerRuleWithContent.<RpcResponse>builder()
                                            .onException()
                                            .thenFailure();
    }
Returns a {@link CircuitBreakerRuleWithContent} which returns {@link CircuitBreakerDecision#failure()}
when there's an {@link Exception} raised.
core/src/test/java/com/linecorp/armeria/client/circuitbreaker/CircuitBreakerRpcClientTest.java