Docs: Add scopes section for Grafana to explain read_user requirement

doc/administration/monitoring/performance/grafana_configuration.md

@@ -74,6 +74,32 @@ GitLab sidebar:
 
 GitLab displays your link in the **Menu > Admin > Monitoring > Metrics Dashboard**.
 
+## Required Scopes
+
+> [Introduced](https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/5822) in GitLab 13.10.
+
+When setting up Grafana through the process above, no scope shows in the screen at
+**Menu >** **{admin}** **Admin > Applications > GitLab Grafana**. However, the `read_user` scope is
+required and is provided to the application automatically. Note that setting any scope other than
+`read_user` without also including `read_user` leads to this error when you try to log in using
+GitLab as the OAuth provider:
+
+```plaintext
+The requested scope is invalid, unknown, or malformed.
+```
+
+If you see this error, make sure that one of the following is true in the GitLab Grafana
+configuration screen:
+
+- No scopes appear.
+- The `read_user` scope is included.
+
+> Versions of GitLab prior 13.10 use the API scope instead of `read_user`. In versions of GitLab
+> prior to 13.10, the API scope:
+>
+> - Is required to access Grafana through the GitLab OAuth provider.
+> - Is set by enabling the Grafana application as shown in [Integration with GitLab UI](#integration-with-gitlab-ui).
+
 ## Security Update
 
 Users running GitLab version 12.0 or later should immediately upgrade to one of the