Add docs for the query variables from URL

doc/user/project/integrations/prometheus.md

@@ -197,6 +197,23 @@ There are 2 methods to specify a variable in a query or dashboard:
 1. Variables can be specified using the [Liquid template format](https://shopify.dev/docs/liquid/reference/basics), for example `{{ci_environment_slug}}` ([added](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/20793) in GitLab 12.6).
 1. You can also enclose it in quotation marks with curly braces with a leading percent, for example `"%{ci_environment_slug}"`. This method is deprecated though and support will be [removed in the next major release](https://gitlab.com/gitlab-org/gitlab/issues/37990).
 
+#### Query Variables from URL
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/214500) in GitLab 13.0.
+
+GitLab supports setting custom variables through URL parameters. Surround the variable
+name with double curly braces (`{{example}}`) to interpolate the variable in a query:
+
+```plaintext
+avg(sum(container_memory_usage_bytes{container_name!="{{pod}}"}) by (job)) without (job)  /1024/1024/1024'
+```
+
+The URL for this query would be:
+
+```plaintext
+http://gitlab.com/<user>/<project>/-/environments/<environment_id>/metrics?dashboard=.gitlab%2Fdashboards%2Fcustom.yml&pod=POD
+```
+
 #### Editing additional metrics from the dashboard
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/208976) in GitLab 12.9.