package main

import (
	"log"
	"net/http"
	"time"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promauto"
	"github.com/prometheus/client_golang/prometheus/promhttp"
)

func main() {
	t := time.Now()

	failedHistogram := promauto.NewHistogramVec(prometheus.HistogramOpts{
		Namespace: "prometheus",
		Subsystem: "poc",
		Name:      "test",
		Buckets:   prometheus.DefBuckets,
		Help:      "my help",
	}, []string{"result"})

	observer := failedHistogram.WithLabelValues("failed")

	withExemplar, ok := observer.(prometheus.ExemplarObserver)
	if !ok {
		panic("could not use observer as an exemplar observer")
	}

	// The following line would yield a valid scrape 
	// withExemplar.ObserveWithExemplar(time.Since(t).Seconds(), prometheus.Labels{"traceID": "1"})

	// The following line is allowed by the client, yet renders an invalid metrics page
	withExemplar.ObserveWithExemplar(time.Since(t).Seconds(), prometheus.Labels{})

	log.Println("Listening on port 8000")
	http.ListenAndServe(":8000", promhttp.InstrumentMetricHandler(
		prometheus.DefaultRegisterer,
		promhttp.HandlerFor(prometheus.DefaultGatherer,
			promhttp.HandlerOpts{
				// Necessary to enable exemplars
				EnableOpenMetrics: true,
			}),
	))
}