chore: update activity metrics (#908)

plugin/metrics/collector.go

@@ -1,6 +1,14 @@
 package metric
 
+// Metric is the API message for metric.
+type Metric struct {
+	ID     string
+	Name   string
+	Labels map[string]string
+}
+
 // Collector is the interface definition for metric collector.
 type Collector interface {
+	Identify(id string) error
 	Collect(metric *Metric) error
 }