As many people have requested and discussed in #3159.

The variable name are kept the same as before. Corresponding command-line flag is named, and description are written, after the existing flag for gRPC.

The scope of this change is intentionally limited to the minimum. That is, I have not yet added `--probe=false`, because it shouldn't be a blocker if we can change the port number.
Signed-off-by: Yusuke KUOKA <ykuoka@gmail.com>