📖 updated missing METRO ENV variables. This can help to take advantag…

…e of automated,

       interconnected bare metal across our global metros.

docs/book/src/user/quick-start.md

@@ -928,6 +928,8 @@ are also a few optional tunables if you'd like to change the OS or CIDRs used.
 export PROJECT_ID="2b59569f-10d1-49a6-a000-c2fb95a959a1"
 # The facility where you want your cluster to be provisioned
 export FACILITY="da11"
+# This can help to take advantage of automated, interconnected bare metal across our global metros.
+export METRO="da"
 # What plan to use for your control plane nodes
 export CONTROLPLANE_NODE_TYPE="m3.small.x86"
 # What plan to use for your worker nodes