main.go

package main

import (
	"fmt"
	"log"
	"net/http"
	"os"

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

var (
	environment = os.Getenv("ENVIRONMENT")

	appVersion string
	version    = prometheus.NewGauge(prometheus.GaugeOpts{
		Name: "version",
		Help: "Version information about this binary",
		ConstLabels: map[string]string{
			"version": appVersion,
		},
	})

	httpRequestsTotal = prometheus.NewCounterVec(prometheus.CounterOpts{
		Name: "http_requests_total",
		Help: "Count of all HTTP requests",
	}, []string{"code", "method"})

	httpRequestDuration = prometheus.NewHistogramVec(prometheus.HistogramOpts{
		Name: "http_request_duration_seconds",
		Help: "Duration of all HTTP requests",
	}, []string{"code", "handler", "method"})

	foundHandler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.WriteHeader(http.StatusOK)
		w.Write([]byte(fmt.Sprintf("New feature: Hello World from %s", environment)))
	})
	notfoundHandler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.WriteHeader(http.StatusNotFound)
	})
	internalErrorHandler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.WriteHeader(http.StatusInternalServerError)
	})
)

var r *prometheus.Registry // Exported Prometheus registry variable

func main() {
	version.Set(1)

	r = prometheus.NewRegistry()
	r.MustRegister(httpRequestsTotal)
	r.MustRegister(httpRequestDuration)
	r.MustRegister(version)

	foundChain := promhttp.InstrumentHandlerDuration(
		httpRequestDuration.MustCurryWith(prometheus.Labels{"handler": "found"}),
		promhttp.InstrumentHandlerCounter(httpRequestsTotal, foundHandler),
	)

	mux := http.NewServeMux()
	mux.Handle("/", foundChain)
	mux.Handle("/err", promhttp.InstrumentHandlerCounter(httpRequestsTotal, notfoundHandler))
	mux.Handle("/internal-err", promhttp.InstrumentHandlerCounter(httpRequestsTotal, internalErrorHandler))
	mux.Handle("/metrics", promhttp.HandlerFor(r, promhttp.HandlerOpts{}))

	srv := &http.Server{Addr: ":8080", Handler: mux}

	log.Fatal(srv.ListenAndServe())
}