Use panel+ref for unique id

Previously, if a user changed the query between
requests, the previous query would not be
canceled. This handles that edge-case.

public/app/features/panel/metrics_panel_ctrl.ts

@@ -183,7 +183,7 @@ class MetricsPanelCtrl extends PanelCtrl {
     };
 
     metricsQuery.targets.forEach(function(target) {
-      target.exprID = target.expr + target.refId + metricsQuery.panelId;
+      target.exprID = target.refId + metricsQuery.panelId;
     });
 
     return datasource.query(metricsQuery);