DATAMONGO-1552 Add $facet, $bucket and $bucketAuto aggregation stages

[mp911de]

We now support multi-faceted aggregation and bucket operations via Aggregations. Bucket aggregation stages can be used standalone to accumulate documents of an aggregation in buckets or together with $facet to create multi-faceted categorizations.

BucketAutoOperation bucketPrice = bucketAuto(Multiply.valueOf("price").multiplyBy(10), 3)
		.withGranularity(Granularities.E12)    
		.andOutputCount().as("count")    
		.andOutput("title").push().as("titles")    
		.andOutputExpression("price * 10")    
		.sum().as("sum");

BucketOperation bucketYear = bucket("year")    
		.withBoundaries(1800, 1900, 2000).withDefaultBucket("Other").andOutputCount().as("count")    
		.andOutput("title").push().as("titles");

newAggregation(Art.class,
		project("title", "artist", "year", "price"),    
		facet(match(Criteria.where("price").exists(true)), bucketPrice).as("categorizeByPrice")    
				.and(bucketYear).as("categorizeByYear"));

translates to:

[
    { "$project": {  "title": 1, "artist": 1, "year": 1, "price": 1 } },
    { "$facet": {
        "categorizeByPrice": [
            { "$match": { "price": {  "$exists": true } } },
            { "$bucketAuto": {
                "buckets": 3,
                "granularity": "E12",
                "groupBy": { "$multiply": [ "$price", 10 ] },
                "output": {
                    "count": { "$sum": 1 },
                    "titles": { "$push": "$title" },
                    "sum": { "$sum": { "$multiply": [ "$price", 10 ] } } }
            } } ],
        "categorizeByYear": [ 
            { "$bucket": {
                "boundaries": [ 1800, 1900, 2000 ],
                "default": "Other",
                "groupBy": "$year",
                "output": {
                    "count": { "$sum": 1 },
                    "titles": { "$push": "$title" }
                } } } ]
        }
    }
]

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Related ticket: DATAMONGO-1552