When logging messages to a file, MongoDB summarizes long log messages rather than truncating because the end of the message often contains valuable information, such as query execution time.
MongoDB should do the same when writing to syslog but use a maximum line length appropriate for syslog.
When a long line is logged, it is truncated to ensure that the logs don't bloat.
However syslog also truncates long lines and has a lower max length (1024 I believe). As a result the ms duration is missing.
This line in in 2.6 should either be configurable or should be syslog aware.