Templating: Makes so __searchFilter can be used as part of regular expression (#20103)

* Fix: Fixes searchfilter wildcard char in regular expressions
Fixes: #20006

* Refactor: Uses TemplateSrv and ScopedVars instead

* Docs: Updates docs with new format

* Tests: Corrects test description

docs/sources/features/datasources/graphite.md

@@ -114,19 +114,25 @@ variable with all possible values that exist in the wildcard position.
 You can also create nested variables that use other variables in their definition. For example
 `apps.$app.servers.*` uses the variable `$app` in its query definition.
 
-#### Using `$__searchFilter` to filter results in Query Variable
+#### Using `__searchFilter` to filter results in Query Variable
 > Available from Grafana 6.5 and above
 
-Using `$__searchFilter` in the query field will filter the query result based on what the user types in the dropdown select box.
-When nothing has been entered by the user the default value for `$__searchFilter` is `*`.
+Using `__searchFilter` in the query field will filter the query result based on what the user types in the dropdown select box.
+When nothing has been entered by the user the default value for `__searchFilter` is `*` and `` when used as part of a regular expression.
 
-The example below shows how to use `$__searchFilter` as part of the query field to enable searching for `server` while the user types in the dropdown select box.
+The example below shows how to use `__searchFilter` as part of the query field to enable searching for `server` while the user types in the dropdown select box.
 
 Query
 ```bash
 apps.$app.servers.$__searchFilter
 ```
 
+TagValues
+```bash
+tag_values(server, server=~${__searchFilter:regex})
+```
+
+
 ### Variable Usage
 
 You can use a variable in a metric node path or as a parameter to a function.