cmd/stdiscorv: Expose build info in metrics

cmd/stdiscosrv/stats.go

@@ -14,6 +14,14 @@ import (
 )
 
 var (
+	buildInfo = prometheus.NewGaugeVec(
+		prometheus.GaugeOpts{
+			Namespace: "syncthing",
+			Subsystem: "discovery",
+			Name:      "build_info",
+			Help:      "A metric with a constant '1' value labeled by version, goversion, builduser and builddate from which stdiscosrv was built.",
+		}, []string{"version", "goversion", "builduser", "builddate"})
+
 	apiRequestsTotal = prometheus.NewCounterVec(
 		prometheus.CounterOpts{
 			Namespace: "syncthing",
@@ -112,7 +120,8 @@ const (
 )
 
 func init() {
-	prometheus.MustRegister(apiRequestsTotal, apiRequestsSeconds,
+	prometheus.MustRegister(buildInfo,
+		apiRequestsTotal, apiRequestsSeconds,
 		lookupRequestsTotal, announceRequestsTotal,
 		replicationSendsTotal, replicationRecvsTotal,
 		databaseKeys, databaseStatisticsSeconds,