Record metrics for invocations when they start

* This experimental patch records metrics as invocations start
so that the metrics can be used to make better scale to zero
decisions in faas-idler.

Tested with Kubernetes on a single-node cluster, metrics
reported as expected. Existing metrics still report.

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

gateway/metrics/exporter.go

@@ -41,6 +41,7 @@ func (e *Exporter) Describe(ch chan<- *prometheus.Desc) {
 	e.metricOptions.GatewayFunctionInvocation.Describe(ch)
 	e.metricOptions.GatewayFunctionsHistogram.Describe(ch)
 	e.metricOptions.ServiceReplicasGauge.Describe(ch)
+	e.metricOptions.StartedCounter.Describe(ch)
 
 	e.metricOptions.ServiceMetrics.Counter.Describe(ch)
 	e.metricOptions.ServiceMetrics.Histogram.Describe(ch)
@@ -51,6 +52,8 @@ func (e *Exporter) Collect(ch chan<- prometheus.Metric) {
 	e.metricOptions.GatewayFunctionInvocation.Collect(ch)
 	e.metricOptions.GatewayFunctionsHistogram.Collect(ch)
 
+	e.metricOptions.StartedCounter.Collect(ch)
+
 	e.metricOptions.ServiceReplicasGauge.Reset()
 	for _, service := range e.services {
 		e.metricOptions.ServiceReplicasGauge.

gateway/metrics/metrics.go

@@ -17,6 +17,7 @@ type MetricOptions struct {
 	GatewayFunctionsHistogram *prometheus.HistogramVec
 	ServiceReplicasGauge      *prometheus.GaugeVec
 	ServiceMetrics            *ServiceMetricOptions
+	StartedCounter            *prometheus.CounterVec
 }
 
 // ServiceMetricOptions provides RED metrics
@@ -81,6 +82,16 @@ func BuildMetricsOptions() MetricOptions {
 		[]string{"method", "path", "status"},
 	)
 
+	startedCounter := prometheus.NewCounterVec(
+		prometheus.CounterOpts{
+			Namespace: "gateway",
+			Subsystem: "function",
+			Name:      "invocation_started",
+			Help:      "The total number of function HTTP requests started.",
+		},
+		[]string{"function_name"},
+	)
+
 	serviceMetricOptions := &ServiceMetricOptions{
 		Counter:   counter,
 		Histogram: histogram,
@@ -91,6 +102,7 @@ func BuildMetricsOptions() MetricOptions {
 		GatewayFunctionInvocation: gatewayFunctionInvocation,
 		ServiceReplicasGauge:      serviceReplicas,
 		ServiceMetrics:            serviceMetricOptions,
+		StartedCounter:            startedCounter,
 	}
 
 	return metricsOptions