stats: add ability to set static prometheus labels

This changes lets you define static Prometheus labels in the config.
This can be useful for tagging your stats with the custom labels you
need for your environment.

    stats:
      type: prometheus
      labels:
        myLabelOne: value1
        myLabelTwo: value2

examples/config.yml

@@ -272,6 +272,10 @@ logging:
   #namespace: prometheusns
   #subsystem: nebula
   #interval: 10s
+  # You can optionally set static labels to include with all metrics
+  #labels:
+  #  myStaticLabel1: value1
+  #  myStaticLabel2: value2
 
   # enables counter metrics for meta packets
   #   e.g.: `messages.tx.handshake`

stats.go

@@ -93,8 +93,19 @@ func startPrometheusStats(l *logrus.Logger, i time.Duration, c *config.C, buildV
 		return nil, fmt.Errorf("stats.path should not be empty")
 	}
 
-	pr := prometheus.NewRegistry()
-	pClient := mp.NewPrometheusProvider(metrics.DefaultRegistry, namespace, subsystem, pr, i)
+	pry := prometheus.NewRegistry()
+	var prr prometheus.Registerer = pry
+
+	labelsRaw := c.GetMap("stats.labels", nil)
+	if labelsRaw != nil {
+		labels := prometheus.Labels{}
+		for k, v := range labelsRaw {
+			labels[fmt.Sprintf("%v", k)] = fmt.Sprintf("%v", v)
+		}
+		prr = prometheus.WrapRegistererWith(labels, prr)
+	}
+
+	pClient := mp.NewPrometheusProvider(metrics.DefaultRegistry, namespace, subsystem, prr, i)
 	if !configTest {
 		go pClient.UpdatePrometheusMetrics()
 	}
@@ -111,14 +122,14 @@ func startPrometheusStats(l *logrus.Logger, i time.Duration, c *config.C, buildV
 			"boringcrypto": strconv.FormatBool(boringEnabled()),
 		},
 	})
-	pr.MustRegister(g)
+	prr.MustRegister(g)
 	g.Set(1)
 
 	var startFn func()
 	if !configTest {
 		startFn = func() {
 			l.Infof("Prometheus stats listening on %s at %s", listen, path)
-			http.Handle(path, promhttp.HandlerFor(pr, promhttp.HandlerOpts{ErrorLog: l}))
+			http.Handle(path, promhttp.HandlerFor(pry, promhttp.HandlerOpts{ErrorLog: l}))
 			log.Fatal(http.ListenAndServe(listen, nil))
 		}
 	}