Normalize empty help values in CollectAndCompare (#1378)

Due to an inconsistency in the text protocol between encoding and
decoding, it was not possible to use the testutil.CollectAndCompare
function to test metrics with empty help values. To fix this, normalize
empty help values from the expected/want side of the test so that they
compare correctly with empty values on the actual/got side of the test.

Signed-off-by: Billy Keyes <bluekeyes@gmail.com>

prometheus/testutil/testutil.go

@@ -47,6 +47,7 @@ import (
 	"github.com/davecgh/go-spew/spew"
 	dto "github.com/prometheus/client_model/go"
 	"github.com/prometheus/common/expfmt"
+	"google.golang.org/protobuf/proto"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/internal"
@@ -230,6 +231,20 @@ func convertReaderToMetricFamily(reader io.Reader) ([]*dto.MetricFamily, error)
 		return nil, fmt.Errorf("converting reader to metric families failed: %w", err)
 	}
 
+	// The text protocol handles empty help fields inconsistently. When
+	// encoding, any non-nil value, include the empty string, produces a
+	// "# HELP" line. But when decoding, the help field is only set to a
+	// non-nil value if the "# HELP" line contains a non-empty value.
+	//
+	// Because metrics in a registry always have non-nil help fields, populate
+	// any nil help fields in the parsed metrics with the empty string so that
+	// when we compare text encodings, the results are consistent.
+	for _, metric := range notNormalized {
+		if metric.Help == nil {
+			metric.Help = proto.String("")
+		}
+	}
+
 	return internal.NormalizeMetricFamilies(notNormalized), nil
 }
 

prometheus/testutil/testutil_test.go

@@ -168,6 +168,26 @@ func TestCollectAndCompareNoLabel(t *testing.T) {
 	}
 }
 
+func TestCollectAndCompareNoHelp(t *testing.T) {
+	const metadata = `
+		# TYPE some_total counter
+	`
+
+	c := prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "some_total",
+	})
+	c.Inc()
+
+	expected := `
+
+		some_total 1
+	`
+
+	if err := CollectAndCompare(c, strings.NewReader(metadata+expected), "some_total"); err != nil {
+		t.Errorf("unexpected collecting result:\n%s", err)
+	}
+}
+
 func TestCollectAndCompareHistogram(t *testing.T) {
 	inputs := []struct {
 		name        string