chore(poller): add metric to track the worker number

Add metric to track multiple task.

poller/metric.go

@@ -0,0 +1,33 @@
+package poller
+
+import "sync/atomic"
+
+// Metric interface
+type Metric interface {
+	IncBusyWorker() uint64
+	DecBusyWorker() uint64
+	BusyWorkers() uint64
+}
+
+var _ Metric = (*metric)(nil)
+
+type metric struct {
+	busyWorkers uint64
+}
+
+// NewMetric for default metric structure
+func NewMetric() Metric {
+	return &metric{}
+}
+
+func (m *metric) IncBusyWorker() uint64 {
+	return atomic.AddUint64(&m.busyWorkers, 1)
+}
+
+func (m *metric) DecBusyWorker() uint64 {
+	return atomic.AddUint64(&m.busyWorkers, ^uint64(0))
+}
+
+func (m *metric) BusyWorkers() uint64 {
+	return atomic.LoadUint64(&m.busyWorkers)
+}