added build info to metrics

Signed-off-by: Kris Budde <Kris.Budde@gmail.com>

exporter.go

@@ -97,4 +97,6 @@ func (e *exporter) Collect(ch chan<- prometheus.Metric) {
 	for _, countervec := range e.queueMetricsCounter {
 		countervec.Collect(ch)
 	}
+
+	BuildInfo.Collect(ch)
 }

version.go

@@ -1,8 +1,29 @@
 package main
 
+import "github.com/prometheus/client_golang/prometheus"
+
 var (
 	Version   string
 	Revision  string
 	Branch    string
 	BuildDate string
 )
+
+//BuildInfo is a metric with a constant '1' value labeled by version, revision, branch and build date on which the rabbitmq_exporter was built
+var BuildInfo *prometheus.GaugeVec
+
+func init() {
+	BuildInfo = newBuildInfo()
+}
+
+func newBuildInfo() *prometheus.GaugeVec {
+	metric := prometheus.NewGaugeVec(
+		prometheus.GaugeOpts{
+			Name: "rabbitmq_exporter_build_info",
+			Help: "A metric with a constant '1' value labeled by version, revision, branch and build date on which the rabbitmq_exporter was built.",
+		},
+		[]string{"version", "revision", "branch", "builddate"},
+	)
+	metric.WithLabelValues(Version, Revision, Branch, BuildDate).Set(1)
+	return metric
+}