Add missing metricNames to error

In to see which metric names are missing, we can add them to the error
message.

Signed-off-by: leonnicolas <leonloechner@gmx.de>

prometheus/testutil/testutil.go

@@ -255,7 +255,20 @@ func compareMetricFamilies(got, expected []*dto.MetricFamily, metricNames ...str
 		got = filterMetrics(got, metricNames)
 		expected = filterMetrics(expected, metricNames)
 		if len(metricNames) > len(got) {
-			return fmt.Errorf("expected metrics name not found")
+			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 {
+					missingMetricNames = append(missingMetricNames, name)
+				}
+			}
+			return fmt.Errorf("expected metric name(s) not found: %v", missingMetricNames)
 		}
 	}
 

prometheus/testutil/testutil_test.go

@@ -383,7 +383,7 @@ func TestScrapeAndCompare(t *testing.T) {
 		some_total2{ label2 = "value2" } 1
 	`,
 			metricNames: []string{"some_total3"},
-			errPrefix:   "expected metrics name not found",
+			errPrefix:   "expected metric name(s) not found",
 			fail:        true,
 		},
 		"one of multiple expected metric names is not scraped": {
@@ -399,7 +399,7 @@ func TestScrapeAndCompare(t *testing.T) {
 		some_total2{ label2 = "value2" } 1
 	`,
 			metricNames: []string{"some_total1", "some_total3"},
-			errPrefix:   "expected metrics name not found",
+			errPrefix:   "expected metric name(s) not found",
 			fail:        true,
 		},
 	}