fix to remove reset in prometheus backend

backend/prometheus.go

@@ -17,17 +17,20 @@ var (
 	camel = regexp.MustCompile("(^[^A-Z0-9]*|[A-Z0-9]*)([A-Z0-9][^A-Z]+|$)")
 )
 
+// Prometheus this holds a list of prometheus gauges which have been created, one for each metric
+// that we want to expose. These are created on the fly as we receive metrics from the agent.
+//
+// Note: these metrics are not unique to a cluster / queue, as these labels are added to the
+// value when it is set.
 type Prometheus struct {
-	totals    map[string]*prometheus.GaugeVec
-	queues    map[string]*prometheus.GaugeVec
-	pipelines map[string]*prometheus.GaugeVec
+	totals map[string]*prometheus.GaugeVec
+	queues map[string]*prometheus.GaugeVec
 }
 
 func NewPrometheusBackend() *Prometheus {
 	return &Prometheus{
-		totals:    make(map[string]*prometheus.GaugeVec),
-		queues:    make(map[string]*prometheus.GaugeVec),
-		pipelines: make(map[string]*prometheus.GaugeVec),
+		totals: make(map[string]*prometheus.GaugeVec),
+		queues: make(map[string]*prometheus.GaugeVec),
 	}
 }
 
@@ -38,16 +41,14 @@ func (p *Prometheus) Serve(path, addr string) {
 	log.Fatal(http.ListenAndServe(addr, m))
 }
 
+// Collect receives a set of metrics from the agent and creates or updates the prometheus gauges
+//
+// Note: This is called once per agent token per interval
 func (p *Prometheus) Collect(r *collector.Result) error {
-	// Clear the gauges to prevent stale values from persisting forever.
-	for _, gauge := range p.queues {
-		gauge.Reset()
-	}
-
 	for name, value := range r.Totals {
 		labelNames := []string{"cluster"}
 		gauge, ok := p.totals[name]
-		if !ok {
+		if !ok { // first time this metric has been seen so create a new gauge
 			gauge = prometheus.NewGaugeVec(prometheus.GaugeOpts{
 				Name: fmt.Sprintf("buildkite_total_%s", camelToUnderscore(name)),
 				Help: fmt.Sprintf("Buildkite Total: %s", name),
@@ -56,14 +57,14 @@ func (p *Prometheus) Collect(r *collector.Result) error {
 			p.totals[name] = gauge
 		}
 
-		// note that r.Cluster will be empty for unclustered agents, this label will be dropped by promethues
+		// note that r.Cluster will be empty for unclustered agents, this label will be dropped by prometheus
 		gauge.WithLabelValues(r.Cluster).Set(float64(value))
 	}
 
 	for queue, counts := range r.Queues {
 		for name, value := range counts {
 			gauge, ok := p.queues[name]
-			if !ok {
+			if !ok { // first time this metric has been seen so create a new gauge
 				labelNames := []string{"queue", "cluster"}
 				gauge = prometheus.NewGaugeVec(prometheus.GaugeOpts{
 					Name: fmt.Sprintf("buildkite_queues_%s", camelToUnderscore(name)),
@@ -73,7 +74,7 @@ func (p *Prometheus) Collect(r *collector.Result) error {
 				p.queues[name] = gauge
 			}
 
-			// note that r.Cluster will be empty for unclustered agents, this label will be dropped by promethues
+			// note that r.Cluster will be empty for unclustered agents, this label will be dropped by prometheus
 			gauge.WithLabelValues(queue, r.Cluster).Set(float64(value))
 		}
 	}

main.go

@@ -19,6 +19,8 @@ import (
 var metricsBackend backend.Backend
 
 func main() {
+	log.SetFlags(log.Ldate | log.Ltime | log.Lshortfile)
+
 	var (
 		interval     = flag.Duration("interval", 0, "Update metrics every interval, rather than once")
 		showVersion  = flag.Bool("version", false, "Show the version")