plugin/proxy: for duration metrics (#1323)

The README.md was updated by not the actual metric.

Fixes #1321

plugin/proxy/proxy.go

@@ -102,7 +102,7 @@ func (p Proxy) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (
 			if backendErr == nil {
 				w.WriteMsg(reply)
 
-				RequestDuration.WithLabelValues(state.Proto(), upstream.Exchanger().Protocol(), familyToString(state.Family()), host.Name).Observe(float64(time.Since(start) / time.Millisecond))
+				RequestDuration.WithLabelValues(state.Proto(), upstream.Exchanger().Protocol(), familyToString(state.Family()), host.Name).Observe(time.Since(start).Seconds())
 
 				return 0, taperr
 			}