Add proxy config note to usage statistics docs

Previously the usage statistics documentation did not mention proxy
configuration. Proxy mis-configuration is a common cause of usage
statistics failing to be reported, which can cause other features
(like DevOps Score) to be non-functional. This adds a brief note about
proxy configuration to the top of the usage statistics documentation.

doc/user/admin_area/settings/usage_statistics.md

@@ -10,10 +10,13 @@ to perform various actions.
 All statistics are opt-out. You can enable/disable them in the
 **Admin Area > Settings > Metrics and profiling** section **Usage statistics**.
 
-NOTE: **Note:**
+## Network configuration
+
 Allow network traffic from your GitLab instance to IP address `104.196.17.203:443`, to send
 usage statistics to GitLab Inc.
 
+If your GitLab instance is behind a proxy, set the appropriate [proxy configuration variables](https://docs.gitlab.com/omnibus/settings/environment-variables.html).
+
 ## Version Check **(CORE ONLY)**
 
 If enabled, version check will inform you if a new version is available and the