Merge pull request #12693 from dehrax/docs-prom-queryvars

Docs: Prometheus template query variables

docs/sources/features/datasources/prometheus.md

@@ -75,6 +75,32 @@ Name | Description
 
 For details of *metric names*, *label names* and *label values* are please refer to the [Prometheus documentation](http://prometheus.io/docs/concepts/data_model/#metric-names-and-labels).
 
+
+#### Using interval and range variables
+
+> Support for `$__range` and `$__range_ms` only available from Grafana v5.3
+
+It's possible to use some global built-in variables in query variables; `$__interval`, `$__interval_ms`, `$__range` and `$__range_ms`, see [Global built-in variables](/reference/templating/#global-built-in-variables) for more information. These can be convenient to use in conjunction with the `query_result` function when you need to filter variable queries since
+`label_values` function doesn't support queries.
+
+Make sure to set the variable's `refresh` trigger to be `On Time Range Change` to get the correct instances when changing the time range on the dashboard.
+
+**Example usage:**
+
+Populate a variable with the the busiest 5 request instances based on average QPS over the time range shown in the dashboard:
+
+```
+Query: query_result(topk(5, sum(rate(http_requests_total[$__range])) by (instance)))
+Regex: /"([^"]+)"/
+```
+
+Populate a variable with the instances having a certain state over the time range shown in the dashboard:
+
+```
+Query: query_result(max_over_time(<metric>[$__range]) != <state>)
+Regex:
+```
+
 ### Using variables in queries
 
 There are two syntaxes:

docs/sources/reference/templating.md

@@ -273,6 +273,12 @@ The `$__timeFilter` is used in the MySQL data source.
 
 This variable is only available in the Singlestat panel and can be used in the prefix or suffix fields on the Options tab. The variable will be replaced with the series name or alias.
 
+### The $__range Variable
+
+> Only available in Grafana v5.3+
+
+Currently only supported for Prometheus data sources. This variable represents the range for the current dashboard. It is calculated by `to - from`. It has a millisecond representation called `$__range_ms`.
+
 ## Repeating Panels
 
 Template variables can be very useful to dynamically change your queries across a whole dashboard. If you want