Add feature flag to instance statistics page

doc/user/admin_area/analytics/instance_statistics.md

 ---
-stage: none
-group: unassigned
+stage: Manage
+group: Analytics
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
 ---
 
@@ -8,6 +8,9 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/235754) in GitLab 13.4.
 
+CAUTION: **Warning:**
+This feature might not be available to you. Check the **version history** note above for details.
+
 Instance Statistics gives you an overview of how much data your instance contains, and how quickly this volume is changing over time.
 
 To see Instance Statistics, go to **Admin Area > Analytics > Instance Statistics**.
@@ -33,3 +36,22 @@ in the categories shown in [Total counts](#total-counts).
 These charts help you visualize how rapidly these records are being created on your instance.
 
 ![Instance Activity Pipelines chart](img/instance_activity_pipelines_chart_v13_6.png)
+
+### Enable or disable Instance Statistics
+
+Instance Statistics is under development and not ready for production use. It is
+deployed behind a feature flag that is **disabled by default**.
+[GitLab administrators with access to the GitLab Rails console](../../../administration/feature_flags.md)
+can enable it.
+
+To enable it:
+
+```ruby
+Feature.enable(:instance_statistics)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:instance_statistics)
+```