plugin/forward: metrics: make docs reflect reality (#3311)

Remove talk about labels that are not added.
Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/forward/README.md

@@ -104,9 +104,8 @@ If monitoring is enabled (via the *prometheus* directive) then the following met
   and we are randomly (this always uses the `random` policy) spraying to an upstream.
 * `coredns_forward_socket_count_total{to}` - number of cached sockets per upstream.
 
-Where `to` is one of the upstream servers (**TO** from the config), `proto` is the protocol used by
-the incoming query ("tcp" or "udp"), and family the transport family ("1" for IPv4, and "2" for
-IPv6).
+Where `to` is one of the upstream servers (**TO** from the config), `rcode` is the returned RCODE
+from the upstream.
 
 ## Examples