provide ace editor for external datasource plugin (#9224)

public/app/plugins/datasource/prometheus/snippets/prometheus.js

@@ -0,0 +1,21 @@
+// jshint ignore: start
+// jscs: disable
+ace.define("ace/snippets/prometheus",["require","exports","module"], function(require, exports, module) {
+"use strict";
+
+// exports.snippetText = "# rate\n\
+// snippet r\n\
+//   rate(${1:metric}[${2:range}])\n\
+// ";
+
+exports.snippets = [
+  {
+    "content": "rate(${1:metric}[${2:range}])",
+    "name": "rate()",
+    "scope": "prometheus",
+    "tabTrigger": "r"
+  }
+];
+
+exports.scope = "prometheus";
+});