Use TemplateSrv for the templating functionality

It turns out that TemplateSrv was passed to the plugin, so it can be
used. So, use that instead of doing the substituting by hand.

One side-effect of that change, which is the reason for it in the
first place, is that now "global variables" are also substituted.

plugin.json

@@ -10,7 +10,7 @@
       "url": "https://redhat.com"
     },
     "keywords": ["singlestat", "link", "panel"],
-    "version": "1.0.0",
+    "version": "2.0.0",
     "updated": "2020-03-05"
   },
 

src/singlevalue_ctrl.js

@@ -2,7 +2,7 @@ import { MetricsPanelCtrl } from 'app/plugins/sdk';
 import { PanelEvents } from '@grafana/data';
 
 class SingleValueCtrl extends MetricsPanelCtrl {
-    constructor($scope, $injector) {
+    constructor($scope, $injector, templateSrv) {
         super($scope, $injector);
 
         const panelDefaults = {
@@ -17,6 +17,7 @@ class SingleValueCtrl extends MetricsPanelCtrl {
             }
         }
 
+        this.templateSrv = templateSrv;
         this.events.on(PanelEvents.panelInitialized, this.adjustFontSize.bind(this));
         this.events.on(PanelEvents.editModeInitialized, this.onInitEditMode.bind(this));
         this.events.on(PanelEvents.dataReceived, this.onDataReceived.bind(this));
@@ -40,19 +41,14 @@ class SingleValueCtrl extends MetricsPanelCtrl {
             return;
         }
 
-        // Imitate the "templating" syntax of the table panel plugin
-        this.text = this.panel.textTemplate.replace(
-            /\${__cell_(\d+)}/g,
-            (match, cellNumber) => (
-                data[cellNumber]
-            )
-        );
-        this.url = this.panel.urlTemplate.replace(
-            /\${__cell_(\d+)}/g,
-            (match, cellNumber) => (
-                data[cellNumber]
-            )
-        );
+        // Substitute variables in the two templates
+        const vars = {};
+        for (let key in data) {
+            let value = data[key];
+            vars["__cell_" + key] = { value: value, text: value ? value.toString() : '' };
+        }
+        this.text = this.templateSrv.replace(this.panel.textTemplate, vars);
+        this.url = this.templateSrv.replace(this.panel.urlTemplate, vars, encodeURIComponent);
     }
 
     render() {