metrics/prometheus: expose metrics in prometheus format too (#17077)

* metrics/prometheus: added prometheus http server and metrics collector

* metrics/prometheus: minor cleanups

* metrics/prometheus: named keys instead name in tag

* metrics/prometheus: minor typo cleanups, sorted report

metrics/exp/exp.go

@@ -9,6 +9,7 @@ import (
 	"sync"
 
 	"github.com/ethereum/go-ethereum/metrics"
+	"github.com/ethereum/go-ethereum/metrics/prometheus"
 )
 
 type exp struct {
@@ -42,6 +43,7 @@ func Exp(r metrics.Registry) {
 	// http.HandleFunc("/debug/vars", e.expHandler)
 	// haven't found an elegant way, so just use a different endpoint
 	http.Handle("/debug/metrics", h)
+	http.Handle("/debug/metrics/prometheus", prometheus.Handler(r))
 }
 
 // ExpHandler will return an expvar powered metrics handler.