Normalize empty help values in CollectAndCompare

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