db.statistics.aggregate([

    { $match: {
        $and: [
        { startingTime : {"$gte": new Date("2017-01-01T00:00:00.000Z") }},
        { master: { $regex: /rhos-qe-jenkins.*/ }},
        { projectName: {$regex: 'DFG.*-rhos'}},
        { result: 'SUCCESS'}
        ]
    }
    },

    { "$project": {
        "duration": "$duration",
        "month": { "$substr": [ "$startingTime", 0, 7 ] }
    }},
    { "$group": {
        "_id": "$month",
        "hours_avg": { "$avg": {$divide: ["$duration", 3600]} },
        "count": { "$sum": 1 }
    }},
    { "$sort": {"_id": 1} }
])

/* 1 */
{
    "_id" : "2017-01",
    "hours_avg" : 0.0,
    "count" : 3.0
}

/* 2 */
{
    "_id" : "2017-02",
    "hours_avg" : 8.66822486772487,
    "count" : 105.0
}

/* 3 */
{
    "_id" : "2017-05",
    "hours_avg" : 0.0,
    "count" : 23.0
}

/* 4 */
{
    "_id" : "2017-06",
    "hours_avg" : 18.3405488051843,
    "count" : 823.0
}

/* 5 */
{
    "_id" : "2017-07",
    "hours_avg" : 19.7344725343321,
    "count" : 89.0
}