middleware/proxy: drop protcol in metric

This blows up prometheus (panic in CoreDNS) and is not that
handy to have.

middleware/proxy/metrics.go

@@ -16,7 +16,7 @@ var (
 		Name:      "request_duration_milliseconds",
 		Buckets:   append(prometheus.DefBuckets, []float64{50, 100, 200, 500, 1000, 2000, 3000, 4000, 5000, 10000}...),
 		Help:      "Histogram of the time (in milliseconds) each request took.",
-	}, []string{"protocol", "zone"})
+	}, []string{"zone"})
 )
 
 // OnStartup sets up the metrics on startup. This is done for all proxy protocols.