chore(config): add MetricsWithoutAuth (#906)

syncthing/syncthing#10045

Author: swenske

users/config.rst

@@ -948,6 +948,9 @@ The following child elements may be present:
     When this setting is disabled, the GUI will not send 401 responses so users
     won't see browser popups prompting for username and password.
 
+.. option:: gui.metricsWithoutAuth
+
+    If true, this allows access to the '/metrics' without authentication.
 
 LDAP Element
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