package metrics

import (
	"net/http"
	"sync"

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

// Handler returns an http.Handler for the local registry.
func Handler() http.Handler {
	promhttp.Handler()
	return promhttp.InstrumentMetricHandler(
		registry,
		promhttp.HandlerFor(registry, promhttp.HandlerOpts{}))
}

var (
	registry = prometheus.NewRegistry()
	// registeredCollectorsMtx protects collectors slice.
	// It should not be acessed concurrently, but we can easily forget this in future, thus this mutex.
	registeredCollectorsMtx sync.Mutex
	registeredCollectors    []prometheus.Collector
)

func init() {
	mustRegister(collectors.NewProcessCollector(collectors.ProcessCollectorOpts{}))
	mustRegister(collectors.NewGoCollector())
}

func mustRegister(cs ...prometheus.Collector) {
	for i := range cs {
		registry.MustRegister(cs[i])
	}
	registeredCollectorsMtx.Lock()
	registeredCollectors = append(registeredCollectors, cs...)
	registeredCollectorsMtx.Unlock()
}