Add CollectAndFormat to testutil, allowing caller to assert as they want to on the exported metric

prometheus/testutil/testutil.go

@@ -183,9 +183,8 @@ func ScrapeAndCompare(url string, expected io.Reader, metricNames ...string) err
 	return compareMetricFamilies(scraped, wanted, metricNames...)
 }
 
-// CollectAndCompare registers the provided Collector with a newly created
-// pedantic Registry. It then calls GatherAndCompare with that Registry and with
-// the provided metricNames.
+// CollectAndCompare collects the metrics identified by `metricNames` and compares them in the Prometheus text
+// exposition format to the data read from expected.
 func CollectAndCompare(c prometheus.Collector, expected io.Reader, metricNames ...string) error {
 	reg := prometheus.NewPedanticRegistry()
 	if err := reg.Register(c); err != nil {
@@ -221,6 +220,31 @@ func TransactionalGatherAndCompare(g prometheus.TransactionalGatherer, expected
 	return compareMetricFamilies(got, wanted, metricNames...)
 }
 
+// CollectAndFormat collects the metrics identified by `metricNames` and returns them in the given format.
+func CollectAndFormat(c prometheus.Collector, format expfmt.FormatType, metricNames ...string) ([]byte, error) {
+	reg := prometheus.NewPedanticRegistry()
+	if err := reg.Register(c); err != nil {
+		return nil, fmt.Errorf("registering collector failed: %w", err)
+	}
+
+	gotFiltered, err := reg.Gather()
+	if err != nil {
+		return nil, fmt.Errorf("gathering metrics failed: %w", err)
+	}
+
+	gotFiltered = filterMetrics(gotFiltered, metricNames)
+
+	var gotFormatted bytes.Buffer
+	enc := expfmt.NewEncoder(&gotFormatted, expfmt.NewFormat(format))
+	for _, mf := range gotFiltered {
+		if err := enc.Encode(mf); err != nil {
+			return nil, fmt.Errorf("encoding gathered metrics failed: %w", err)
+		}
+	}
+
+	return gotFormatted.Bytes(), nil
+}
+
 // convertReaderToMetricFamily would read from a io.Reader object and convert it to a slice of
 // dto.MetricFamily.
 func convertReaderToMetricFamily(reader io.Reader) ([]*dto.MetricFamily, error) {

prometheus/testutil/testutil_test.go

@@ -20,6 +20,8 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/prometheus/common/expfmt"
+
 	"github.com/prometheus/client_golang/prometheus"
 )
 
@@ -431,3 +433,32 @@ func TestCollectAndCount(t *testing.T) {
 		t.Errorf("unexpected metric count, got %d, want %d", got, want)
 	}
 }
+
+func TestCollectAndFormat(t *testing.T) {
+	const expected = `# HELP foo_bar A value that represents the number of bars in foo.
+# TYPE foo_bar counter
+foo_bar{fizz="bang"} 1
+`
+	c := prometheus.NewCounterVec(
+		prometheus.CounterOpts{
+			Name: "foo_bar",
+			Help: "A value that represents the number of bars in foo.",
+		},
+		[]string{"fizz"},
+	)
+	c.WithLabelValues("bang").Inc()
+
+	got, err := CollectAndFormat(c, expfmt.TypeTextPlain, "foo_bar")
+	if err != nil {
+		t.Errorf("unexpected error: %s", err.Error())
+	}
+
+	gotS := string(got)
+	if err != nil {
+		t.Errorf("unexpected error: %s", err.Error())
+	}
+
+	if gotS != expected {
+		t.Errorf("unexpected metric output, got %q, expected %q", gotS, expected)
+	}
+}