Add nginxexporter_build_info metric

Author: pleshakov

README.md

@@ -79,6 +79,8 @@ Usage of ./nginx-prometheus-exporter:
 
 ### Exported Metrics
 
+* Common metrics:
+    * `nginxexporter_build_info` -- shows the exporter build information.
 * For NGINX, the following metrics are exported:
     * All [stub_status](http://nginx.org/en/docs/http/ngx_http_stub_status_module.html) metrics.
     * `nginx_up` -- shows the status of the last metric scrape: `1` for a successful scrape and `0` for a failed one.

exporter.go

@@ -70,6 +70,20 @@ func main() {
 
 	registry := prometheus.NewRegistry()
 
+	buildInfoMetric := prometheus.NewGauge(
+		prometheus.GaugeOpts{
+			Name: "nginxexporter_build_info",
+			Help: "Exporter build information",
+			ConstLabels: prometheus.Labels{
+				"version":   version,
+				"gitCommit": gitCommit,
+			},
+		},
+	)
+	buildInfoMetric.Set(1)
+
+	registry.MustRegister(buildInfoMetric)
+
 	httpClient := &http.Client{
 		Timeout: *timeout,
 		Transport: &http.Transport{