DefaultHandshakeHandler logs client errors on error level, for example

level: error
logger: DefaultHandshakeHandler 
message: Handshake failed due to invalid Upgrade header: null

It is wrong. Correct application behavior - reject malformed handshake - is reported as an error.
I monitor application error in log to detect wrong behavior and fix them.
If level error contains both wrong behavior and correct behavior, levelling has no sense at all.