start collecting RTT and bandwidth metrics when Collect is calleed

metrics.go

@@ -54,8 +54,9 @@ func init() {
 }
 
 type aggregatingCollector struct {
-	mutex sync.Mutex
+	cronOnce sync.Once
 
+	mutex                sync.Mutex
 	highestID            uint64
 	conns                map[uint64] /* id */ *tracingConn
 	rtts                 prometheus.Histogram
@@ -80,7 +81,6 @@ func newAggregatingCollector() *aggregatingCollector {
 			Buckets: prometheus.ExponentialBuckets(1, 1.5, 40), // 1s to ~12 weeks
 		}),
 	}
-	go c.cron()
 	return c
 }
 
@@ -144,6 +144,8 @@ func (c *aggregatingCollector) cron() {
 }
 
 func (c *aggregatingCollector) Collect(metrics chan<- prometheus.Metric) {
+	c.cronOnce.Do(func() { go c.cron() })
+
 	c.mutex.Lock()
 	defer c.mutex.Unlock()