Request duration metric is not populated if request resolution was fa…

…ulty (#257)

resolver/metrics_resolver.go

@@ -31,15 +31,22 @@ func (m *MetricsResolver) Resolve(request *Request) (*Response, error) {
 			"client": strings.Join(request.ClientNames, ","),
 			"type":   dns.TypeToString[request.Req.Question[0].Qtype]}).Inc()
 
+		reqDurationMs := float64(time.Since(request.RequestTS).Milliseconds())
+		responseType := "err"
+
+		if response != nil {
+			responseType = response.RType.String()
+		}
+
+		m.durationHistogram.WithLabelValues(responseType).Observe(reqDurationMs)
+
 		if err != nil {
 			m.totalErrors.Inc()
 		} else {
 			m.totalResponse.With(prometheus.Labels{
 				"reason":        response.Reason,
 				"response_code": dns.RcodeToString[response.Res.Rcode],
 				"response_type": response.RType.String()}).Inc()
-			reqDurationMs := float64(time.Since(request.RequestTS).Milliseconds())
-			m.durationHistogram.WithLabelValues(response.RType.String()).Observe(reqDurationMs)
 		}
 	}