Fix: prometheus monitor_status metric has 4 values

The prometheus monitor_status metric has actually 4 values. This can easily be verified by looking up the related source code or by using the metric in grafana an see values like 2 (which indicates timeout).

server/prometheus.js

@@ -28,7 +28,7 @@ const monitorResponseTime = new PrometheusClient.Gauge({
 
 const monitorStatus = new PrometheusClient.Gauge({
     name: "monitor_status",
-    help: "Monitor Status (1 = UP, 0= DOWN)",
+    help: "Monitor Status (1 = UP, 0= DOWN, 2= PENDING, 3= MAINTENANCE)",
     labelNames: commonLabels
 });