final CircuitBreakerRule rule() {
        checkState(rule != null, "rule is not set.");
        return rule;
    }
Returns the {@link CircuitBreakerRule}.

@throws IllegalStateException if the {@link CircuitBreakerRule} is not set
core/src/main/java/com/linecorp/armeria/client/circuitbreaker/AbstractCircuitBreakerClient.java