https://mongodb.github.io/mongo-csharp-driver/2.7/apidocs/html/M_MongoDB_Driver_IAggregateFluent_1_Bucket__1.htm
IAggregateFluent(TResult).Bucket(TValue) Method (AggregateExpressionDefinition(TResult, TValue),...
Appends a $bucket stage to the pipeline.
buckettvaluemethod