For monitoring logs - Analog: www.analog.cx. It's a bit heavy to set up (lots of config options) but pretty powerful. You can set it up to write small, machine readable files that feed in to your stats server (such as Zabbix or Nagios).
Bear in mind, though, that your web logs might not report the bytes served, they might just report the size of the file requested. (Not used nginx, but I know apache does this.) If you serve a lot of big downloads, then this can lead to an enormous over-estimation of bandwidth, if you consider many download managers will fire off 16 or 32 requests for the same file...
Apache has a logio module, which sacrifices some of the immediacy of logging for a true count of bytes served.