Add metric counter for queued requests in throttling control behavior (alibaba#431)

* Add `flow_wait_total` counter

Author: HobbyBear

core/flow/slot.go

@@ -17,6 +17,7 @@ package flow
 import (
 	"github.com/alibaba/sentinel-golang/core/base"
 	"github.com/alibaba/sentinel-golang/core/stat"
+	metric_exporter "github.com/alibaba/sentinel-golang/exporter/metric"
 	"github.com/alibaba/sentinel-golang/logging"
 	"github.com/alibaba/sentinel-golang/util"
 	"github.com/pkg/errors"
@@ -27,9 +28,17 @@ const (
 )
 
 var (
-	DefaultSlot = &Slot{}
+	DefaultSlot   = &Slot{}
+	flowWaitCount = metric_exporter.NewCounter(
+		"flow_wait_total",
+		"Flow wait count",
+		[]string{"resource"})
 )
 
+func init() {
+	metric_exporter.Register(flowWaitCount)
+}
+
 type Slot struct {
 }
 
@@ -58,6 +67,7 @@ func (s *Slot) Check(ctx *base.EntryContext) *base.TokenResult {
 		}
 		if r.Status() == base.ResultStatusShouldWait {
 			if nanosToWait := r.NanosToWait(); nanosToWait > 0 {
+				flowWaitCount.Add(float64(ctx.Input.BatchCount), ctx.Resource.Name())
 				// Handle waiting action.
 				util.Sleep(nanosToWait)
 			}

exporter/metric/empty_exporter.go

@@ -0,0 +1,65 @@
+package metric
+
+import "net/http"
+
+type EmptyExporter struct {
+}
+
+func (e *EmptyExporter) HTTPHandler() http.Handler {
+	return nil
+}
+
+type emptyMetric struct {
+}
+
+func (e emptyMetric) Register() error {
+	return nil
+}
+
+func (e emptyMetric) Unregister() bool {
+	return false
+}
+
+func (e emptyMetric) Reset() {
+	return
+}
+
+func newEmptyExporter() *EmptyExporter {
+	return &EmptyExporter{}
+}
+
+type emptyCounter struct {
+	emptyMetric
+}
+
+func (e emptyCounter) Add(value float64, labelValues ...string) {
+	return
+}
+
+func (e *EmptyExporter) NewCounter(name, desc string, labelNames []string) Counter {
+	return emptyCounter{}
+}
+
+type emptyGauge struct {
+	emptyMetric
+}
+
+func (e emptyGauge) Set(value float64, labelValues ...string) {
+	return
+}
+
+func (e *EmptyExporter) NewGauge(name, desc string, labelNames []string) Gauge {
+	return &emptyGauge{}
+}
+
+type emptyHistogram struct {
+	emptyMetric
+}
+
+func (e emptyHistogram) Observe(value float64, labelValues ...string) {
+	return
+}
+
+func (e *EmptyExporter) NewHistogram(name, desc string, buckets []float64, labelNames []string) Histogram {
+	return &emptyHistogram{}
+}

exporter/metric/exporter.go

@@ -33,7 +33,11 @@ var (
 )
 
 func init() {
-	exporter = newPrometheusExporter()
+	if config.MetricExportHTTPAddr() != "" {
+		exporter = newPrometheusExporter()
+	} else {
+		exporter = newEmptyExporter()
+	}
 
 	host, _ = os.Hostname()
 	if host == "" {