histogramvec

// Create a new descriptor with a label.
desc := prometheus.NewDesc(
	"fridge_temperature_celsius",
	"The temperature of the fridge in each store",
	[]string{"store_name"},
	nil,
)

// Create a HistogramVec.
c := Config{
	BucketLimits: []float64{0, 1, 2, 3, 4, 5},
}
hv, _ := New(c)

// Define a struct to hold sensor data,
// here, the temperature of each store's refrigerator.
type StoreTemperature struct {
	StoreName   string
	Temperature float64
}

// Read data from an external system.
firstSamples := []StoreTemperature{
	{
		StoreName:   "London",
		Temperature: 2.5,
	},
	{
		StoreName:   "Frankfurt",
		Temperature: 3.2,
	},
}

// Add the samples to the HistogramVec.
for _, s := range firstSamples {
	_ = hv.Add(s.StoreName, s.Temperature)
}

// Emit each metric, such as in a Prometheus Collector.
// This will emit Histograms for both the London and Frankfurt stores.
for label, h := range hv.Histograms() {
	metric := prometheus.MustNewConstHistogram(
		desc,
		h.Count(), h.Sum(), h.Buckets(),
		label,
	)

	fmt.Println(metric)
}

// Read data again from an external system.
// For example, this would be a separate invocation of Collect
// in a Prometheus Collector.
secondSamples := []StoreTemperature{
	{
		StoreName:   "London",
		Temperature: 2.6,
	},
	// Note: Frankfurt store has shut down,
	// and does not report fridge temperatures anymore.
}

// Add the second set of sample to the HistogramVec.
for _, s := range secondSamples {
	_ = hv.Add(s.StoreName, s.Temperature)
}

// Ensure that any stores that have shut down are removed from the HistogramVec.
// This should be done on every sampling (ommitted earlier for clarity).
storeNames := []string{}
for _, s := range secondSamples {
	storeNames = append(storeNames, s.StoreName)
}
hv.Ensure(storeNames)

// Again, emit each metric. This will only emit a Histogram for the London store,
// as the Frankfurt store has been removed.
for label, h := range hv.Histograms() {
	metric := prometheus.MustNewConstHistogram(
		desc,
		h.Count(), h.Sum(), h.Buckets(),
		label,
	)

	fmt.Println(metric)
}