OC-9346: log_msg

 
Add log_msg to annotations as "msg"

 - Changed default ````log_msg```` to ````undefined````
  - If ````log_msg```` remains undefined when assembling the request log, then
    do not annotate the log with msg
  - Otherwise, use ````{raw, Msg}```` so we can see the Erlang term
  - This is a change from the old fast_log behavior. In the old
    behavior, if ````log_msg```` is not changed, then a ````""```` will be emitted into
    the log line as a ````[]````. This is not a useful behavior and there is no
    known metric that depends on it.

Note: This PR does not contain the Ruby Depsolver work