Add log_msg to annotations as "msg"

  - Changed defeault 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.