go-micro broker 消息的订阅和发布

　　broker是用于消息的订阅和发布，即异步处理，不过通常会使用队列方式替代，很少使用go-micro的broker

　　默认是http方式

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// Broker is an interface used for asynchronous messaging. type Broker interface {     Options() Options     Address() string     Connect() error     Disconnect() error     Init(...Option) error     Publish(string, *Message, ...PublishOption) error     Subscribe(string, Handler, ...SubscribeOption) (Subscriber, error)     String() string }   // Handler is used to process messages via a subscription of a topic. // The handler is passed a publication interface which contains the // message and optional Ack method to acknowledge receipt of the message. type Handler func(Publication) error   type Message struct {     Header map[string]string     Body   []byte }   // Publication is given to a subscription handler for processing type Publication interface {     Topic() string     Message() *Message     Ack() error }   // Subscriber is a convenience return type for the Subscribe method type Subscriber interface {     Options() SubscribeOptions     Topic() string     Unsubscribe() error }