Merge branch 'docs-aqualls-add-intro-line' into 'master'

Templated variables: add introduced-in link

See merge request gitlab-org/gitlab!36152

doc/user/project/integrations/prometheus.md

@@ -754,6 +754,8 @@ When a query returns too many data points, the heatmap data bucket dimensions te
 
 ### Templating variables for metrics dashboards
 
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/214539) in GitLab 13.0.
+
 Templating variables can be used to make your metrics dashboard more versatile.
 
 #### Templating variable types