Bug: testutil package unexpected results for Histograms

[philipgough]

@beorn7 this follows up from our chat in #497

I'm seeing similar behaviour as reported in #494 with Counters. This time with Histograms. I've written the following failing test

func TestCollectAndCompareHistogram(t *testing.T) {
	inputs := []struct {
		name        string
		c           prometheus.Collector
		metadata    string
		expect      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{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,
		},
	}

	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)
			}
		})

	}
}

It returns the following output:

FAIL
    --- FAIL: TestCollectAndCompareHistogram/Testing_Histogram_Collector (0.00s)
        testutil_test.go:237: unexpected collecting result:
            
            metric output does not match expectation; want:
            
            # HELP some_histogram An example of a histogram
            # TYPE some_histogram histogram
            some_histogram_bucket{le="1"} 0
            some_histogram_bucket{le="2"} 0
            some_histogram_bucket{le="3"} 1
            some_histogram_bucket{le="+Inf"} 1
            some_histogram_sum 2.5
            some_histogram_count 1
            
            
            got:
            
            # HELP some_histogram An example of a histogram
            # TYPE some_histogram histogram
            some_histogram_bucket{le="1"} 0
            some_histogram_bucket{le="2"} 0
            some_histogram_bucket{le="3"} 1
            some_histogram_bucket{le="+Inf"} 1
            some_histogram_sum 2.5
            some_histogram_count 1

Comparing the want and the got strings:

name:"some_histogram" help:"An example of a histogram" type:HISTOGRAM metric:<histogram:<sample_count:1 sample_sum:2.5 bucket:<cumulative_count:0 upper_bound:1 > bucket:<cumulative_count:0 upper_bound:2 > bucket:<cumulative_count:1 upper_bound:3 > > > 
name:"some_histogram" help:"An example of a histogram" type:HISTOGRAM metric:<histogram:<sample_count:1 sample_sum:2.5 bucket:<cumulative_count:0 upper_bound:1 > bucket:<cumulative_count:0 upper_bound:2 > bucket:<cumulative_count:1 upper_bound:3 > bucket:<cumulative_count:1 upper_bound:inf > > >

[beorn7]

I see, that's the +Inf bucket, which is redundant/optional. Hmmm, I think I'll just go one step further and compare the text format directly. Currently, in real-life Prometheus settings, the text format is the one Prometheus uses anyway, so it's most relevant.

Will create another PR at my next convenience.