...
{ "_id" : ObjectId("579fc863d246f4640173d0b4"),
  "flow" : [ 178488283, NumberLong("4294967295"), NumberLong("4294967295"),
     11, 0, 2, 264,
     1470073539, 1470073539, 48471, 5678, 0, 17, 0, 0, 0, 0, 0 ],
  "sequence" : 64754, "sensor" :  "192.168.236.1" }
...

db.collector.aggregate(
  [{ $group: { _id: 'rx' , bytes : { $sum : 'flow.6' }, count : { $sum :1 }}}] )

{ "_id" : "rx", "bytes" : 0, "count" : 151291 }

> db.collector.aggregate( [ {$unwind: "$flow" },
 { $group: { _id: 'rx' , bytes : { $sum : 6 }, count : { $sum :1 }}}] )
{ "_id" : "rx", "bytes" : 17328276, "count" : 2888046 }