Fix metric comparison for empty labels

reflect.DeepEqual is not suitable for zero occurrences of repeated
proto messages. This changes the comparison to act on the string
representation of proto messages.

Signed-off-by: beorn7 <beorn@soundcloud.com>

prometheus/testutil/testutil.go

@@ -37,7 +37,6 @@ import (
 	"bytes"
 	"fmt"
 	"io"
-	"reflect"
 
 	"github.com/prometheus/common/expfmt"
 
@@ -125,49 +124,56 @@ func CollectAndCompare(c prometheus.Collector, expected io.Reader, metricNames .
 // exposition format. If any metricNames are provided, only metrics with those
 // names are compared.
 func GatherAndCompare(g prometheus.Gatherer, expected io.Reader, metricNames ...string) error {
-	metrics, err := g.Gather()
+	got, err := g.Gather()
 	if err != nil {
 		return fmt.Errorf("gathering metrics failed: %s", err)
 	}
 	if metricNames != nil {
-		metrics = filterMetrics(metrics, metricNames)
+		got = filterMetrics(got, metricNames)
 	}
 	var tp expfmt.TextParser
-	expectedMetrics, err := tp.TextToMetricFamilies(expected)
+	wantRaw, err := tp.TextToMetricFamilies(expected)
 	if err != nil {
 		return fmt.Errorf("parsing expected metrics failed: %s", err)
 	}
+	want := internal.NormalizeMetricFamilies(wantRaw)
 
-	if !reflect.DeepEqual(metrics, internal.NormalizeMetricFamilies(expectedMetrics)) {
-		// Encode the gathered output to the readable text format for comparison.
-		var buf1 bytes.Buffer
-		enc := expfmt.NewEncoder(&buf1, expfmt.FmtText)
-		for _, mf := range metrics {
-			if err := enc.Encode(mf); err != nil {
-				return fmt.Errorf("encoding result failed: %s", err)
-			}
+	if len(got) != len(want) {
+	}
+	for i := range got {
+		if got[i].String() != want[i].String() {
+			return notMatchingError(got, want)
 		}
-		// Encode normalized expected metrics again to generate them in the same ordering
-		// the registry does to spot differences more easily.
-		var buf2 bytes.Buffer
-		enc = expfmt.NewEncoder(&buf2, expfmt.FmtText)
-		for _, mf := range internal.NormalizeMetricFamilies(expectedMetrics) {
-			if err := enc.Encode(mf); err != nil {
-				return fmt.Errorf("encoding result failed: %s", err)
-			}
+	}
+	return nil
+}
+
+// notMatchingError encodes both provided slices of metric families into the
+// text format and creates a readable error message from the result.
+func notMatchingError(got, want []*dto.MetricFamily) error {
+	var gotBuf, wantBuf bytes.Buffer
+	enc := expfmt.NewEncoder(&gotBuf, expfmt.FmtText)
+	for _, mf := range got {
+		if err := enc.Encode(mf); err != nil {
+			return fmt.Errorf("encoding gathered metrics failed: %s", err)
+		}
+	}
+	enc = expfmt.NewEncoder(&wantBuf, expfmt.FmtText)
+	for _, mf := range want {
+		if err := enc.Encode(mf); err != nil {
+			return fmt.Errorf("encoding expected metrics failed: %s", err)
 		}
+	}
 
-		return fmt.Errorf(`
+	return fmt.Errorf(`
 metric output does not match expectation; want:
 
 %s
 
 got:
 
 %s
-`, buf2.String(), buf1.String())
-	}
-	return nil
+`, wantBuf.String(), gotBuf.String())
 }
 
 func filterMetrics(metrics []*dto.MetricFamily, names []string) []*dto.MetricFamily {