Golangでlogのタイムスタンプをマイクロ秒単位にする方法

最初に結論

logのタイムスタンプをマイクロ秒単位にするには、log.SetFlags(log.Lmicroseconds) を実行しておけばよい。
https://golang.org/pkg/log/#pkg-constants

例

デフォルトのlog

package main

import (
    "log"
    "time"
)

func main() {
    count := 0
    for {
        time.Sleep(100 * time.Millisecond)
        log.Printf("count=%d\n", count)
        count++
    }
}

$ go run a1.go
2018/01/22 10:58:34 count=0
2018/01/22 10:58:34 count=1
2018/01/22 10:58:34 count=2
2018/01/22 10:58:34 count=3
2018/01/22 10:58:34 count=4
2018/01/22 10:58:34 count=5
2018/01/22 10:58:34 count=6
2018/01/22 10:58:34 count=7
2018/01/22 10:58:35 count=8
2018/01/22 10:58:35 count=9
2018/01/22 10:58:35 count=10
2018/01/22 10:58:35 count=11
2018/01/22 10:58:35 count=12
2018/01/22 10:58:35 count=13
2018/01/22 10:58:35 count=14

log.SetFlags(log.Lmicroseconds)した場合

package main

import (
    "log"
    "time"
)

func main() {
    log.SetFlags(log.Lmicroseconds)　// <-- これを追加
    count := 0
    for {
        time.Sleep(100 * time.Millisecond)
        log.Printf("count=%d\n", count)
        count++
    }
}

$ go run a2.go
10:59:40.963792 count=0
10:59:41.064928 count=1
10:59:41.166225 count=2
10:59:41.267365 count=3
10:59:41.367690 count=4
10:59:41.468041 count=5
10:59:41.569157 count=6
10:59:41.670192 count=7
10:59:41.771581 count=8
10:59:41.871852 count=9
10:59:41.972876 count=10
10:59:42.074049 count=11
10:59:42.174963 count=12