feat: add wal write system call metrics observation

Signed-off-by: Qiuyu Wu <qiuyu.wu@shopee.com>

server/wal/encoder.go

@@ -20,6 +20,7 @@ import (
 	"io"
 	"os"
 	"sync"
+	"time"
 
 	"go.etcd.io/etcd/pkg/v3/crc"
 	"go.etcd.io/etcd/pkg/v3/ioutil"
@@ -92,7 +93,9 @@ func (e *encoder) encode(rec *walpb.Record) error {
 	if padBytes != 0 {
 		data = append(data, make([]byte, padBytes)...)
 	}
+	start := time.Now()
 	n, err = e.bw.Write(data)
+	walWriteSec.Observe(time.Since(start).Seconds())
 	walWriteBytes.Add(float64(n))
 	return err
 }
@@ -118,7 +121,9 @@ func (e *encoder) flush() error {
 func writeUint64(w io.Writer, n uint64, buf []byte) error {
 	// http://golang.org/src/encoding/binary/binary.go
 	binary.LittleEndian.PutUint64(buf, n)
+	start := time.Now()
 	nv, err := w.Write(buf)
+	walWriteSec.Observe(time.Since(start).Seconds())
 	walWriteBytes.Add(float64(nv))
 	return err
 }

server/wal/metrics.go

@@ -28,6 +28,15 @@ var (
 		Buckets: prometheus.ExponentialBuckets(0.001, 2, 14),
 	})
 
+	walWriteSec = prometheus.NewHistogram(prometheus.HistogramOpts{
+		Namespace: "etcd",
+		Subsystem: "disk",
+		Name:      "wal_write_duration_seconds",
+		Help:      "The latency distributions of write called by WAL.",
+
+		Buckets: prometheus.ExponentialBuckets(0.001, 2, 14),
+	})
+
 	walWriteBytes = prometheus.NewGauge(prometheus.GaugeOpts{
 		Namespace: "etcd",
 		Subsystem: "disk",