@UnstableApi
    default boolean isCircuitBreakerException(Exception ex) {
        return ex instanceof FailFastException;
    }
Determines if the given {@link Exception} is related to a circuit breaker.
This method is invoked by the {@link CircuitBreakerClient} to determine if a
fallback logic should be executed.
core/src/main/java/com/linecorp/armeria/client/circuitbreaker/CircuitBreakerClientHandler.java