plugin/metrics: add 'server' label (#1682)

* plugin/metrics: add 'server' label

This uses the new WithServer(ctx) to get the current server from the
context.

First in a larger refactor to make all plugins do this.

* compile

* compile

* lala test

* compile and test

* typos

* Dont duplicate the code

plugin/metrics/handler.go

@@ -25,7 +25,7 @@ func (m *Metrics) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 	rw := dnstest.NewRecorder(w)
 	status, err := plugin.NextOrFailure(m.Name(), m.Next, ctx, rw, r)
 
-	vars.Report(state, zone, rcode.ToString(rw.Rcode), rw.Len, rw.Start)
+	vars.Report(ctx, state, zone, rcode.ToString(rw.Rcode), rw.Len, rw.Start)
 
 	return status, err
 }