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/prometheus/prometheus.go

@@ -0,0 +1,68 @@
+// Copyright 2019 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+// Package prometheus exposes go-metrics into a Prometheus format.
+package prometheus
+
+import (
+	"fmt"
+	"net/http"
+	"sort"
+
+	"github.com/ethereum/go-ethereum/log"
+	"github.com/ethereum/go-ethereum/metrics"
+)
+
+// Handler returns an HTTP handler which dump metrics in Prometheus format.
+func Handler(reg metrics.Registry) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// Gather and pre-sort the metrics to avoid random listings
+		var names []string
+		reg.Each(func(name string, i interface{}) {
+			names = append(names, name)
+		})
+		sort.Strings(names)
+
+		// Aggregate all the metris into a Prometheus collector
+		c := newCollector()
+
+		for _, name := range names {
+			i := reg.Get(name)
+
+			switch m := i.(type) {
+			case metrics.Counter:
+				c.addCounter(name, m.Snapshot())
+			case metrics.Gauge:
+				c.addGauge(name, m.Snapshot())
+			case metrics.GaugeFloat64:
+				c.addGaugeFloat64(name, m.Snapshot())
+			case metrics.Histogram:
+				c.addHistogram(name, m.Snapshot())
+			case metrics.Meter:
+				c.addMeter(name, m.Snapshot())
+			case metrics.Timer:
+				c.addTimer(name, m.Snapshot())
+			case metrics.ResettingTimer:
+				c.addResettingTimer(name, m.Snapshot())
+			default:
+				log.Warn("Unknown Prometheus metric type", "type", fmt.Sprintf("%T", i))
+			}
+		}
+		w.Header().Add("Content-Type", "text/plain")
+		w.Header().Add("Content-Length", fmt.Sprint(c.buff.Len()))
+		w.Write(c.buff.Bytes())
+	})
+}