Add events support in client

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

stats.go

@@ -3,9 +3,10 @@ package containerd
 import "github.com/rcrowley/go-metrics"
 
 var (
-	ContainerStartTimer = metrics.NewTimer()
-	ContainersCounter   = metrics.NewCounter()
-	EventsCounter       = metrics.NewCounter()
+	ContainerStartTimer    = metrics.NewTimer()
+	ContainersCounter      = metrics.NewCounter()
+	EventsCounter          = metrics.NewCounter()
+	EventSubscriberCounter = metrics.NewCounter()
 )
 
 func Metrics() map[string]interface{} {
@@ -13,5 +14,6 @@ func Metrics() map[string]interface{} {
 		"container-start-time": ContainerStartTimer,
 		"containers":           ContainersCounter,
 		"events":               EventsCounter,
+		"events-subscribers":   EventSubscriberCounter,
 	}
 }