"inner-dest plugin username not found": You need to use 3.6+ of syslog-ng
If you get the message:
Error parsing afmongodb, inner-dest plugin username not found
in syslog-ng when trying to connect to mongodb, it is likely that you are running a version of syslog-ng that has support for mongodb, but not support for mongodb authentication. An example of such a versions is 3.5.3-1, which is as of this writing the one in the standard repositories for Ubuntu 14.04LTS. Version 3.6+ are supposed to have support for mongo authentication.
Revision: 3.5.3-1 [@9695e81] (Ubuntu/14.04)"