admin api: add metrics_require_token config option and update doc

doc/book/reference-manual/configuration.md

@@ -777,10 +777,10 @@ with the exception of the metrics endpoint (see `metrics_token`).
 You can use any random string for this value. We recommend generating a random
 token with `openssl rand -base64 32`.
 
-For Garage version earlier than v2.0, if this token is not set,
+For Garage version earlier than `v2.0`, if this token is not set,
 access to these endpoints is disabled entirely.
 
-Since Garage v2.0, additional admin API tokens can be defined dynamically
+Since Garage `v2.0`, additional admin API tokens can be defined dynamically
 in your Garage cluster using administration commands. This new admin token system
 is more flexible since it allows admin tokens to have an expiration date,
 and to have a scope restricted to certain admin API functions. If `admin_token`