plugin/prometheus: write rcode properly to the metrics (#5126)

* plugin/prometheus: write rcode properly to the metrics

Signed-off-by: Ondřej Benkovský <ondrej.benkovsky@jamf.com>

plugin/metrics/recorder.go

@@ -24,7 +24,5 @@ func (r *Recorder) WriteMsg(res *dns.Msg) error {
 	_, r.Caller[0], _, _ = runtime.Caller(1)
 	_, r.Caller[1], _, _ = runtime.Caller(2)
 	_, r.Caller[2], _, _ = runtime.Caller(3)
-	r.Len += res.Len()
-	r.Msg = res
-	return r.ResponseWriter.WriteMsg(res)
+	return r.Recorder.WriteMsg(res)
 }

plugin/metrics/recorder_test.go

@@ -0,0 +1,68 @@
+package metrics
+
+import (
+	"testing"
+
+	"github.com/coredns/coredns/plugin/test"
+	
+	"github.com/miekg/dns"
+)
+
+type inmemoryWriter struct{
+	test.ResponseWriter
+	written []byte
+}
+
+func (r *inmemoryWriter) WriteMsg(m *dns.Msg) error {
+	r.written, _ = m.Pack()
+	return r.ResponseWriter.WriteMsg(m)
+}
+
+func (r *inmemoryWriter) Write(buf []byte) (int, error) {
+	r.written = buf
+	return r.ResponseWriter.Write(buf)
+}
+
+func TestRecorder_WriteMsg(t *testing.T) {
+	successResp := dns.Msg{}
+	successResp.Answer = []dns.RR{
+		test.A("a.example.org. 	1800	IN	A 127.0.0.53"),
+	}
+
+	nxdomainResp := dns.Msg{}
+	nxdomainResp.Rcode = dns.RcodeNameError
+
+	tests := []struct {
+		name      string
+		msg *dns.Msg
+	}{
+		{
+			name: "should record successful response",
+			msg: &successResp,
+		},
+		{
+			name: "should record nxdomain response",
+			msg: &nxdomainResp,
+		},
+	}
+	for i, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tw := inmemoryWriter{ResponseWriter: test.ResponseWriter{}}
+			rec := NewRecorder(&tw)
+
+			if err := rec.WriteMsg(tt.msg); err != nil {
+				t.Errorf("Test %d: WriteMsg() unexpected error %v", i, err)
+			}
+
+			if rec.Msg != tt.msg {
+				t.Errorf("Test %d: Expected value %v for msg, but got %v", i, tt.msg, rec.Msg)
+			}
+			if rec.Len != tt.msg.Len() {
+				t.Errorf("Test %d: Expected value %d for len, but got %d", i, tt.msg.Len(), rec.Len)
+			}
+			if rec.Rcode != tt.msg.Rcode {
+				t.Errorf("Test %d: Expected value %d for rcode, but got %d", i, tt.msg.Rcode, rec.Rcode)
+			}
+		})
+	}
+}