Exposes bug #498 - unexpected results for Histograms in testutil

Signed-off-by: PhilipGough <philip.p.gough@gmail.com>

prometheus/testutil/testutil_test.go

@@ -165,6 +165,82 @@ func TestCollectAndCompareNoLabel(t *testing.T) {
 	}
 }
 
+func TestCollectAndCompareHistogram(t *testing.T) {
+	inputs := []struct {
+		name        string
+		c           prometheus.Collector
+		metadata    string
+		expect      string
+		labels      []string
+		observation float64
+	}{
+		{
+			name: "Testing Histogram Collector",
+			c: prometheus.NewHistogram(prometheus.HistogramOpts{
+				Name:    "some_histogram",
+				Help:    "An example of a histogram",
+				Buckets: []float64{1, 2, 3},
+			}),
+			metadata: `
+				# HELP some_histogram An example of a histogram
+				# TYPE some_histogram histogram
+			`,
+			expect: `
+				some_histogram{le="1"} 0
+				some_histogram{le="2"} 0
+				some_histogram{le="3"} 1
+        			some_histogram_bucket{le="+Inf"} 1
+        			some_histogram_sum 2.5
+        			some_histogram_count 1
+
+			`,
+			observation: 2.5,
+		},
+		{
+			name: "Testing HistogramVec Collector",
+			c: prometheus.NewHistogramVec(prometheus.HistogramOpts{
+				Name:    "some_histogram",
+				Help:    "An example of a histogram",
+				Buckets: []float64{1, 2, 3},
+			}, []string{"test"}),
+
+			metadata: `
+				# HELP some_histogram An example of a histogram
+				# TYPE some_histogram histogram
+			`,
+			expect: `
+            			some_histogram_bucket{test="test",le="1"} 0
+            			some_histogram_bucket{test="test",le="2"} 0
+            			some_histogram_bucket{test="test",le="3"} 1
+            			some_histogram_bucket{test="test",le="+Inf"} 1
+            			some_histogram_sum{test="test"} 2.5
+           		 	some_histogram_count{test="test"} 1
+		
+			`,
+			observation: 2.5,
+		},
+	}
+
+	for _, input := range inputs {
+		switch collector := input.c.(type) {
+		case prometheus.Histogram:
+			collector.Observe(input.observation)
+		case *prometheus.HistogramVec:
+			collector.WithLabelValues("test").Observe(input.observation)
+		default:
+			t.Fatalf("unsuported collector tested")
+
+		}
+
+		t.Run(input.name, func(t *testing.T) {
+			if err := CollectAndCompare(input.c, strings.NewReader(input.metadata+input.expect)); err != nil {
+				t.Errorf("unexpected collecting result:\n%s", err)
+			}
+		})
+
+	}
+}
+
 func TestNoMetricFilter(t *testing.T) {
 	const metadata = `
 		# HELP some_total A value that represents a counter.