add observer

observer/observer.go

@@ -0,0 +1,22 @@
+package observer
+
+import "context"
+
+type Options struct{}
+
+type Option func(opts *Options)
+
+type Observer interface {
+	Observe(ctx context.Context, events []Event, opts ...Option) error
+}
+
+type EventType string
+
+const (
+	EventStatus EventType = "status"
+	EventStats  EventType = "stats"
+)
+
+type Event interface {
+	Type() EventType
+}