stackdriver: add documentation for the template query editor

docs/sources/features/datasources/stackdriver.md

@@ -158,9 +158,9 @@ Example Result: `compute.googleapis.com/instance/cpu/usage_time - server1-prod`
 
 It is also possible to resolve the name of the Monitored Resource Type. 
 
-| Alias Pattern Format     | Description                                     | Example Result   |
-| ------------------------ | ------------------------------------------------| ---------------- |
-| `{{resource.type}}`      | returns the name of the monitored resource type | `gce_instance`     |
+| Alias Pattern Format | Description                                     | Example Result |
+| -------------------- | ----------------------------------------------- | -------------- |
+| `{{resource.type}}`  | returns the name of the monitored resource type | `gce_instance` |
 
 Example Alias By: `{{resource.type}} - {{metric.type}}`
 
@@ -177,7 +177,17 @@ types of template variables.
 
 ### Query Variable
 
-Writing variable queries is not supported yet.
+Variable of the type *Query* allows you to query Stackdriver for various types of data. The Stackdriver data source plugin provides the following `Query Types`.
+
+| Name                | Description                                                                                       |
+| ------------------- | ------------------------------------------------------------------------------------------------- |
+| *Mectric Types*     | Returns a list of metric type names that are available for the specified service.                 |
+| *Metric Labels*     | Returns a list of label values for the `metric label` in the specified metric.                    |
+| *Resource Labels*   | Returns a list of label values for the `resource label` in the specified metric.                  |
+| *Resource Types*    | Returns a list of resource types for the the specified metric.                                    |
+| *Aggregations*      | Returns a list of aggregations (cross series reducers) for the the specified metric.              |
+| *Aligners*          | Returns a list of aligners (per series aligners) for the the specified metric.                    |
+| *Alignment periods* | Returns a list of all alignment periods that are available in Stackdriver query editor in Grafana |
 
 ### Using variables in queries