admin api: add metrics_require_token config option and update doc

doc/book/reference-manual/configuration.md

@@ -808,9 +808,9 @@ You can use any random string for this value. We recommend generating a random t
 #### `metrics_require_token` (since `v2.0.0`) {#admin_metrics_require_token}
 
 If this is set to `true`, accessing the metrics endpoint will always require
-an access token, either the value of `metrics_token` if it is set,
-or an admin API token defined dynamicaly in Garage which has
-the `Metrics` endpoint in its scope.
+an access token. Valid tokens include the `metrics_token` if it is set,
+and admin API token defined dynamicaly in Garage which have
+the `Metrics` endpoint in their scope.
 
 #### `trace_sink` {#admin_trace_sink}