Instrument async handlers

- instruments async handler for report and for queueing async
requests
- make MustRegister only ever run once to prevent sync issues

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>

gateway/metrics/exporter.go

@@ -37,6 +37,7 @@ func NewExporter(options MetricOptions, credentials *auth.BasicAuthCredentials)
 
 // Describe is to describe the metrics for Prometheus
 func (e *Exporter) Describe(ch chan<- *prometheus.Desc) {
+
 	e.metricOptions.GatewayFunctionInvocation.Describe(ch)
 	e.metricOptions.GatewayFunctionsHistogram.Describe(ch)
 	e.metricOptions.ServiceReplicasGauge.Describe(ch)

gateway/metrics/exporter_test.go

@@ -40,24 +40,27 @@ func Test_Describe_DescribesThePrometheusMetrics(t *testing.T) {
 
 	go exporter.Describe(ch)
 
-	d := (<-ch)
+	d := <-ch
 	expectedGatewayFunctionInvocationDesc := `Desc{fqName: "gateway_function_invocation_total", help: "Individual function metrics", constLabels: {}, variableLabels: [function_name code]}`
 	actualGatewayFunctionInvocationDesc := d.String()
 	if expectedGatewayFunctionInvocationDesc != actualGatewayFunctionInvocationDesc {
 		t.Errorf("Want %s, got: %s", expectedGatewayFunctionInvocationDesc, actualGatewayFunctionInvocationDesc)
 	}
-	d = (<-ch)
+
+	d = <-ch
 	expectedGatewayFunctionsHistogramDesc := `Desc{fqName: "gateway_functions_seconds", help: "Function time taken", constLabels: {}, variableLabels: [function_name]}`
 	actualGatewayFunctionsHistogramDesc := d.String()
 	if expectedGatewayFunctionsHistogramDesc != actualGatewayFunctionsHistogramDesc {
 		t.Errorf("Want %s, got: %s", expectedGatewayFunctionsHistogramDesc, actualGatewayFunctionsHistogramDesc)
 	}
-	d = (<-ch)
+
+	d = <-ch
 	expectedServiceReplicasGaugeDesc := `Desc{fqName: "gateway_service_count", help: "Docker service replicas", constLabels: {}, variableLabels: [function_name]}`
 	actualServiceReplicasGaugeDesc := d.String()
 	if expectedServiceReplicasGaugeDesc != actualServiceReplicasGaugeDesc {
 		t.Errorf("Want %s, got: %s", expectedServiceReplicasGaugeDesc, actualServiceReplicasGaugeDesc)
 	}
+
 }
 
 func Test_Collect_CollectsTheNumberOfReplicasOfAService(t *testing.T) {

gateway/metrics/metrics.go

@@ -5,6 +5,7 @@ package metrics
 
 import (
 	"net/http"
+	"sync"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
@@ -24,6 +25,16 @@ type ServiceMetricOptions struct {
 	Counter   *prometheus.CounterVec
 }
 
+// Synchronize to make sure MustRegister only called once
+var once = sync.Once{}
+
+// RegisterExporter registers with Prometheus for tracking
+func RegisterExporter(exporter *Exporter) {
+	once.Do(func() {
+		prometheus.MustRegister(exporter)
+	})
+}
+
 // PrometheusHandler Bootstraps prometheus for metrics collection
 func PrometheusHandler() http.Handler {
 	return promhttp.Handler()
@@ -84,8 +95,3 @@ func BuildMetricsOptions() MetricOptions {
 
 	return metricsOptions
 }
-
-// RegisterExporter registers with Prometheus for tracking
-func RegisterExporter(exporter *Exporter) {
-	prometheus.MustRegister(exporter)
-}