Apply suggestions from code review

Co-authored-by: Bartlomiej Plotka <bwplotka@gmail.com>
Signed-off-by: leonnicolas <60091705+leonnicolas@users.noreply.github.com>

prometheus/testutil/testutil.go

@@ -255,16 +255,9 @@ func compareMetricFamilies(got, expected []*dto.MetricFamily, metricNames ...str
 		got = filterMetrics(got, metricNames)
 		expected = filterMetrics(expected, metricNames)
 		if len(metricNames) > len(got) {
-			h := make(map[string]struct{})
-			for _, mf := range got {
-				if mf == nil {
-					continue
-				}
-				h[mf.GetName()] = struct{}{}
-			}
 			var missingMetricNames []string
 			for _, name := range metricNames {
-				if _, ok := h[name]; !ok {
+			        if _, ok := hasMetricByName(got,name); !ok {
 					missingMetricNames = append(missingMetricNames, name)
 				}
 			}

prometheus/testutil/testutil_test.go

@@ -335,8 +335,8 @@ func TestScrapeAndCompare(t *testing.T) {
 	scenarios := map[string]struct {
 		want        string
 		metricNames []string
-		errPrefix   string
-		fail        bool
+		// expectedErrPrefix if empty, means no fail is expected for the comparison.
+		expectedErrPrefix   string
 	}{
 		"empty metric Names": {
 			want: `