Add scheduledToStart latency for activity (#720)

Author: longquanzheng

internal/common/metrics/constants.go

@@ -56,6 +56,7 @@ const (
 	ActivityPollNoTaskCounter          = CadenceMetricsPrefix + "activity-poll-no-task"
 	ActivityPollSucceedCounter         = CadenceMetricsPrefix + "activity-poll-succeed"
 	ActivityPollLatency                = CadenceMetricsPrefix + "activity-poll-latency"
+	ActivityScheduledToStartLatency    = CadenceMetricsPrefix + "activity-scheduled-to-start-latency"
 	ActivityExecutionFailedCounter     = CadenceMetricsPrefix + "activity-execution-failed"
 	ActivityExecutionLatency           = CadenceMetricsPrefix + "activity-execution-latency"
 	ActivityResponseLatency            = CadenceMetricsPrefix + "activity-response-latency"

internal/internal_task_pollers.go

@@ -809,6 +809,10 @@ func (atp *activityTaskPoller) poll() (*activityTask, error) {
 
 	atp.metricsScope.Counter(metrics.ActivityPollSucceedCounter).Inc(1)
 	atp.metricsScope.Timer(metrics.ActivityPollLatency).Record(time.Now().Sub(startTime))
+
+	scheduledTime := time.Unix(0, response.GetScheduledTimestampOfThisAttempt())
+	atp.metricsScope.Timer(metrics.ActivityScheduledToStartLatency).Record(time.Now().Sub(scheduledTime))
+
 	return &activityTask{task: response, pollStartTime: startTime}, nil
 }