db.b.explain().aggregate([
  {$match: { master: 1} },
  {$group: {"_id":"$status", "counter":{$sum:1}}},
  {$group: {
    "_id": null,
    "pending":{$sum: {$cond:[{$lte:["$_id",99]},"$count",0]}},
    "running":{$sum: {$cond:[{$gte:["$_id",100]},"$count",0]}}
  }}
]);