remember prometheus instance and expose it

in preperation for #2491,#680 and #898

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;