public Builder<T> withListeners(List<? extends ExecutionListener<?, T>> listeners) {
            if (this.listeners == null) {
                this.listeners = new LinkedList<ExecutionListener<?, T>>(listeners);
            } else {
                this.listeners.addAll((Collection) listeners);
            }
            return this;
        }
A command that is used to produce the Observable from the load balancer execution. The load balancer is responsible for
the following:

<ul>
<li>Choose a server</li>
<li>Invoke the {@link com.netflix.loadbalancer.Server} onSubscribe call method</li>
<li>Invoke the {@link ExecutionListener} if any</li>
<li>Retry on exception, controlled by {@link com.netflix.client.RetryHandler}</li>
<li>Provide feedback to the {@link com.netflix.loadbalancer.LoadBalancerStats}</li>
</ul>

@author Allen Wang
ribbon-loadbalancer/src/main/java/com/netflix/loadbalancer/reactive/LoadBalancerCommand.java