rpc: metrics for JSON-RPC method calls (ethereum#20847)

This adds a couple of metrics for tracking the timing
and frequency of method calls:

- rpc/requests gauge counts all requests
- rpc/success gauge counts requests which return err == nil
- rpc/failure gauge counts requests which return err != nil
- rpc/duration/all timer tracks timing of all requests
- rpc/duration/<method>/<success/failure> tracks per-method timing

rpc/handler.go

@@ -327,8 +327,22 @@ func (h *handler) handleCall(cp *callProc, msg *jsonrpcMessage) *jsonrpcMessage
 	if err != nil {
 		return msg.errorResponse(&invalidParamsError{err.Error()})
 	}
-
-	return h.runMethod(cp.ctx, msg, callb, args)
+	start := time.Now()
+	answer := h.runMethod(cp.ctx, msg, callb, args)
+
+	// Collect the statistics for RPC calls if metrics is enabled.
+	// We only care about pure rpc call. Filter out subscription.
+	if callb != h.unsubscribeCb {
+		rpcRequestGauge.Inc(1)
+		if answer.Error != nil {
+			failedReqeustGauge.Inc(1)
+		} else {
+			successfulRequestGauge.Inc(1)
+		}
+		rpcServingTimer.UpdateSince(start)
+		newRPCServingTimer(msg.Method, answer.Error == nil).UpdateSince(start)
+	}
+	return answer
 }
 
 // handleSubscribe processes *_subscribe method calls.

rpc/metrics.go

@@ -0,0 +1,39 @@
+// Copyright 2020 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 rpc
+
+import (
+	"fmt"
+
+	"github.com/ethereum/go-ethereum/metrics"
+)
+
+var (
+	rpcRequestGauge        = metrics.NewRegisteredGauge("rpc/requests", nil)
+	successfulRequestGauge = metrics.NewRegisteredGauge("rpc/success", nil)
+	failedReqeustGauge     = metrics.NewRegisteredGauge("rpc/failure", nil)
+	rpcServingTimer        = metrics.NewRegisteredTimer("rpc/duration/all", nil)
+)
+
+func newRPCServingTimer(method string, valid bool) metrics.Timer {
+	flag := "success"
+	if !valid {
+		flag = "failure"
+	}
+	m := fmt.Sprintf("rpc/duration/%s/%s", method, flag)
+	return metrics.GetOrRegisterTimer(m, nil)
+}