remember prometheus instance and expose it

in preperation for #2491,#680 and #898

server/model/monitor.js

@@ -200,7 +200,7 @@ class Monitor extends BeanModel {
         let previousBeat = null;
         let retries = 0;
 
-        let prometheus = new Prometheus(this);
+        this.prometheus = new Prometheus(this);
 
         const beat = async () => {
 
@@ -729,7 +729,7 @@ class Monitor extends BeanModel {
             await R.store(bean);
 
             log.debug("monitor", `[${this.name}] prometheus.update`);
-            prometheus.update(bean, tlsInfo);
+            this.prometheus.update(bean, tlsInfo);
 
             previousBeat = bean;
 
@@ -814,15 +814,15 @@ class Monitor extends BeanModel {
         clearTimeout(this.heartbeatInterval);
         this.isStop = true;
 
-        this.prometheus().remove();
+        this.prometheus.remove();
     }
 
     /**
-     * Get a new prometheus instance
-     * @returns {Prometheus}
+     * Get prometheus instance
+     * @returns {Prometheus|undefined}
      */
-    prometheus() {
-        return new Prometheus(this);
+    getPrometheus() {
+        return this.prometheus;
     }
 
     /**

server/server.js

@@ -674,9 +674,6 @@ let needSetup = false;
                     throw new Error("Permission denied.");
                 }
 
-                // Reset Prometheus labels
-                server.monitorList[monitor.id]?.prometheus()?.remove();
-
                 bean.name = monitor.name;
                 bean.type = monitor.type;
                 bean.url = monitor.url;