prometheus editor: variable autocomplete support (PR #9988)

* prometheus ace editor, template variable support

* minor fix

* support [[var]] notation

* don't remove last ] if token type is string

* Revert "don't remove last ] if token type is string"

This reverts commit bce5b2d56e.

* Revert "support [[var]] notation"

This reverts commit 10012f8ffe.

* fix token type and regex

public/app/plugins/datasource/prometheus/mode-prometheus.js

@@ -50,6 +50,9 @@ var PrometheusHighlightRules = function() {
       token : "keyword.control",
       regex : "by|without|on|ignoring|group_left|group_right",
       next  : "start-label-list-matcher"
+    }, {
+      token : "variable",
+      regex : "\\$[A-Za-z0-9_]+"
     }, {
       token : keywordMapper,
       regex : "[a-zA-Z_:][a-zA-Z0-9_:]*"