Add Circuit Breaker docs (#390)

message/router/middleware/circuit_breaker.go

@@ -5,16 +5,22 @@ import (
 	"github.com/sony/gobreaker"
 )
 
+// CircuitBreaker is a middleware that wraps the handler in a circuit breaker.
+// Based on the configuration, the circuit breaker will fail fast if the handler keeps returning errors.
+// This is useful for preventing cascading failures.
 type CircuitBreaker struct {
 	cb *gobreaker.CircuitBreaker
 }
 
+// NewCircuitBreaker returns a new CircuitBreaker middleware.
+// Refer to the gobreaker documentation for the available settings.
 func NewCircuitBreaker(settings gobreaker.Settings) CircuitBreaker {
 	return CircuitBreaker{
 		cb: gobreaker.NewCircuitBreaker(settings),
 	}
 }
 
+// Middleware returns the CircuitBreaker middleware.
 func (c CircuitBreaker) Middleware(h message.HandlerFunc) message.HandlerFunc {
 	return func(msg *message.Message) ([]*message.Message, error) {
 		out, err := c.cb.Execute(func() (interface{}, error) {