Add dns64 plugin (#3534)

* Add dns64 plugin

Add external plugin to core in-tree.
* Pull code from upstream: https://github.com/serverwentdown/dns64
* Update docs.

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

* Make dns64 consistent.

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

* Cleanup README

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

* Cleanup minor issues.

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

* Remove proxy method.

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

* dns64: big cleanup

* Make the code a bit more idiomatic
* Add tests
* use proper Upstream API

Signed-off-by: Casey Callendrello <c1@caseyc.net>
Signed-off-by: Ben Kochie <superq@gmail.com>

* A little more clenaup

* Fix some docs.
* Use the correct plugin register method.
* Cleanup some review items.

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

* Add metrics counter for DNS64 translations

Add a basic counter of how many DNS64 translations have been completed.

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

* Add DNSSEC bug link

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

* Test cleanup

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

* dns64: more test cleanup

Signed-off-by: Casey Callendrello <c1@caseyc.net>

Co-authored-by: Casey Callendrello <c1@caseyc.net>

plugin/dns64/metrics.go

@@ -0,0 +1,17 @@
+package dns64
+
+import (
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+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",
+		Help:      "Counter of DNS requests translated by dns64.",
+	}, []string{"server"})
+)