Installed cloud logging ageng on GCE VM instance and send log to StackDriver logging, then Japanese log is not displayed

[cocoa-maemae]

Event

I installed cloud logging agent on GCE VM instance. Sending logs to StackDriver logging is successful, however, Japanese(non-ASCII characters) logs will be disappeared on StackDriver logging. Not character corruption, only they will be empty on StackDriver logging. English logs will be displayed normally.
I checked on GCE VM instance, and then Japanese(non-ASCII characters) logs were displayed normally on the sending target log file. Are non-ASCII characters removed during sending to StackDriver? If that, I'd like to support non-ASCII characters as well.

Environment

GCE VM instance OS: Debian
cloud logging agent config like the below.

<source>
  @type tail
  format none
  path $log_path
  pos_file $pos_path
  read_from_head true
  tag $tag_name
</source>

[zhuqiyu]

<filter **>
@type record_modifier
char_encoding utf-8