nit: remove double validation on folder when register flag

Signed-off-by: SamYuan1990 <yy19902439@126.com>

Author: SamYuan1990

config/config.go

@@ -237,8 +237,8 @@ func RegisterFlags(app *kingpin.Application) ConfigUpdaterFn {
 	logLevel := app.Flag(LogLevelFlag, "Logging level: debug, info, warn, error").Default("info").Enum("debug", "info", "warn", "error")
 	logFormat := app.Flag(LogFormatFlag, "Logging format: text or json").Default("text").Enum("text", "json")
 	// host
-	hostSysFS := app.Flag(HostSysFSFlag, "Host sysfs path").Default("/sys").ExistingDir()
-	hostProcFS := app.Flag(HostProcFSFlag, "Host procfs path").Default("/proc").ExistingDir()
+	hostSysFS := app.Flag(HostSysFSFlag, "Host sysfs path").Default("/sys").String()
+	hostProcFS := app.Flag(HostProcFSFlag, "Host procfs path").Default("/proc").String()
 
 	// monitor
 	monitorInterval := app.Flag(MonitorIntervalFlag,