Update more metrics names (#3799)

The subsystem of a metric must be it's plugin name. "dns" is reserved
for the core. Use a const convention to avoid copy-paste mistakes in
naming.

Fix dns64 and acl plugin.

Signed-off-by: Ben Kochie <superq@gmail.com>

plugin/dns64/metrics.go

@@ -10,8 +10,8 @@ var (
 	// RequestsTranslatedCount is the number of DNS requests translated by dns64.
 	RequestsTranslatedCount = prometheus.NewCounterVec(prometheus.CounterOpts{
 		Namespace: plugin.Namespace,
-		Subsystem: "dns",
-		Name:      "requests_dns64_translated_total",
+		Subsystem: pluginName,
+		Name:      "requests_translated_total",
 		Help:      "Counter of DNS requests translated by dns64.",
 	}, []string{"server"})
 )