Remove monitor server and instead use callback

monitor/item.go

@@ -12,6 +12,10 @@ type Duration struct {
 	time.Duration
 }
 
+func (d *Duration) MarshalJSON() ([]byte, error) {
+	return json.Marshal(d.Duration)
+}
+
 func (d *Duration) UnmarshalJSON(b []byte) error {
 	var v interface{}
 	if err := json.Unmarshal(b, &v); err != nil {