log.SetOutput(logplug.NewJSONPlug(os.Stderr,
	logplug.LogFlag(log.LstdFlags),
	logplug.Hooks(
		logplug.LevelHook(logplug.LevelConfig{
			Levels: []logplug.Level{"DBG", "INFO", "WARN", "ERR"},
			Min:    "INFO",
		}),
	)))

log.Printf("[INFO]output test")
// output: {"level":"INFO","message":"output test"}

// key and value in [] in prefix are used as custom field
log.SetPrefix("[label:test]")
log.Printf("[DBG] debug log")
// output: {"label":"test","level":"DBG","message":"debug log"}