Add tracing option (#487)

Adds a middleware to enable tracing with OpenTracing/OpenZipkin.
Enabling tracing will have a large impact on performance so it is
not advisable in production.

core/dnsserver/directives.go

@@ -75,6 +75,7 @@ func RegisterDevDirective(name, before string) {
 var directives = []string{
 	"root",
 	"bind",
+	"trace",
 	"health",
 	"pprof",