Export build information as metric.

There is more build information that could be put into labels, but
Version, Revision, and Branch seemed like the right level of detail to
me.

version/info.go

@@ -13,6 +13,8 @@
 
 package version
 
+import "github.com/prometheus/client_golang/prometheus"
+
 // Build information. Populated at build-time.
 var (
 	Version   string
@@ -32,3 +34,16 @@ var Map = map[string]string{
 	"buildDate": BuildDate,
 	"goVersion": GoVersion,
 }
+
+func init() {
+	buildInfo := prometheus.NewGaugeVec(
+		prometheus.GaugeOpts{
+			Name: "prometheus_build_info",
+			Help: "A metric with a constant '1' value labeled by version, revision, and branch from which Prometheus was built.",
+		},
+		[]string{"version", "revision", "branch"},
+	)
+	buildInfo.WithLabelValues(Version, Revision, Branch).Set(1)
+
+	prometheus.MustRegister(buildInfo)
+}