chore: systematic syncthing_build_info metric

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/build/build.go

@@ -16,8 +16,18 @@ import (
 	"strconv"
 	"strings"
 	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
 )
 
+var buildInfo = promauto.NewGaugeVec(prometheus.GaugeOpts{
+	Namespace: "syncthing",
+	Subsystem: "build",
+	Name:      "info",
+	Help:      "A metric with a constant '1' value labeled by version information from when the binary was built.",
+}, []string{"program", "version", "codename", "goversion", "builduser", "builddate", "tags"})
+
 const Codename = "Hafnium Hornet"
 
 var (
@@ -98,9 +108,22 @@ func LongVersionFor(program string) string {
 	date := Date.UTC().Format("2006-01-02 15:04:05 MST")
 	v := fmt.Sprintf(`%s %s "%s" (%s %s-%s) %s@%s %s`, program, Version, Codename, runtime.Version(), runtime.GOOS, runtime.GOARCH, User, Host, date)
 
-	if tags := TagsList(); len(tags) > 0 {
+	tags := TagsList()
+	if len(tags) > 0 {
 		v = fmt.Sprintf("%s [%s]", v, strings.Join(tags, ", "))
 	}
+
+	buildInfo.Reset()
+	buildInfo.With(prometheus.Labels{
+		"program":   program,
+		"version":   Version,
+		"codename":  Codename,
+		"goversion": runtime.Version(),
+		"builduser": fmt.Sprintf("%s@%s", User, Host),
+		"builddate": Date.UTC().Format("2006-01-02 15:04:05 MST"),
+		"tags":      strings.Join(tags, ","),
+	}).Set(1)
+
 	return v
 }