plugin/forward Add rcode and rtype to request_duration_seconds metric (#4391)

* plugin/forward Add rcode and rtype to request_duration_seconds metric

Signed-off-by: Maxime Ginters <maxime.ginters@shopify.com>

* Control the cardinality of query type

Signed-off-by: Maxime Ginters <maxime.ginters@shopify.com>

plugin/pkg/dnsutil/monitor.go

@@ -0,0 +1,37 @@
+package dnsutil
+
+import (
+	"github.com/miekg/dns"
+)
+
+var monitorType = map[uint16]struct{}{
+	dns.TypeAAAA:   {},
+	dns.TypeA:      {},
+	dns.TypeCNAME:  {},
+	dns.TypeDNSKEY: {},
+	dns.TypeDS:     {},
+	dns.TypeMX:     {},
+	dns.TypeNSEC3:  {},
+	dns.TypeNSEC:   {},
+	dns.TypeNS:     {},
+	dns.TypePTR:    {},
+	dns.TypeRRSIG:  {},
+	dns.TypeSOA:    {},
+	dns.TypeSRV:    {},
+	dns.TypeTXT:    {},
+	// Meta Qtypes
+	dns.TypeIXFR: {},
+	dns.TypeAXFR: {},
+	dns.TypeANY:  {},
+}
+
+const other = "other"
+
+// QTypeMonitorLabel returns dns type label based on a list of monitored types.
+// Will return "other" for unmonitored ones.
+func QTypeMonitorLabel(qtype uint16) string {
+	if _, known := monitorType[qtype]; known {
+		return dns.Type(qtype).String()
+	}
+	return other
+}