new Prometheus metrics build_info (#3558)

Signed-off-by: Nicolas Lamirault <nicolas.lamirault@gmail.com>

cmd/dex/serve.go

@@ -50,6 +50,15 @@ type serveOptions struct {
 	grpcAddr      string
 }
 
+var buildInfo = prometheus.NewGaugeVec(
+	prometheus.GaugeOpts{
+		Name:      "build_info",
+		Namespace: "dex",
+		Help:      "A metric with a constant '1' value labeled by version from which Dex was built.",
+	},
+	[]string{"version", "go_version", "platform"},
+)
+
 func commandServe() *cobra.Command {
 	options := serveOptions{}
 
@@ -117,6 +126,10 @@ func runServe(options serveOptions) error {
 	logger.Info("config issuer", "issuer", c.Issuer)
 
 	prometheusRegistry := prometheus.NewRegistry()
+
+	prometheusRegistry.MustRegister(buildInfo)
+	recordBuildInfo()
+
 	err = prometheusRegistry.Register(collectors.NewGoCollector())
 	if err != nil {
 		return fmt.Errorf("failed to register Go runtime metrics: %v", err)
@@ -684,3 +697,8 @@ func loadTLSConfig(certFile, keyFile, caFile string, baseConfig *tls.Config) (*t
 	}
 	return loadedConfig, nil
 }
+
+// recordBuildInfo publishes information about Dex version and runtime info through an info metric (gauge).
+func recordBuildInfo() {
+	buildInfo.WithLabelValues(version, runtime.Version(), fmt.Sprintf("%s/%s", runtime.GOOS, runtime.GOARCH)).Set(1)
+}