add: CollectAndFormat to testutil

CollectAndFormat is a helper function that returns the formatted metrics
to the caller, allowing them to use it how they want. This is different
to CollectAndCompare where the comparison is done strictly on behalf of
the caller. Often it is more convenient to perform a simple substring
match on the formatted metric.

Signed-off-by: Jack Cassidy <j.cassidy45@gmail.com>

prometheus/testutil/testutil.go

@@ -221,6 +221,33 @@ func TransactionalGatherAndCompare(g prometheus.TransactionalGatherer, expected
 	return compareMetricFamilies(got, wanted, metricNames...)
 }
 
+// CollectAndFormat registers the provided Collector with a newly created pedantic Registry. It then calls Gather with
+// that Registry and filters by the provided metricNames. It writes the gathered metrics to a bugger formatted according
+// to the provided format argument
+func CollectAndFormat(c prometheus.Collector, format expfmt.FormatType, metricNames ...string) (*bytes.Buffer, 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, 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

@@ -15,11 +15,14 @@ package testutil
 
 import (
 	"fmt"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"strings"
 	"testing"
 
+	"github.com/prometheus/common/expfmt"
+
 	"github.com/prometheus/client_golang/prometheus"
 )
 
@@ -431,3 +434,32 @@ 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, err := io.ReadAll(got)
+	if err != nil {
+		t.Errorf("unexpected error: %s", err.Error())
+	}
+
+	if string(gotS) != expected {
+		t.Errorf("unexpected metric output, got %q, expected %q", string(gotS), expected)
+	}
+}