code: public static <TExpression> Bson group(@Nullable final TExpression id, final List<BsonField> fieldAccumulators) {
          return new GroupStage<TExpression>(id, fieldAccumulators);
      }
docs: Creates a $group pipeline stage for the specified filter
  
  @param <TExpression>     the expression type
  @param id                the id expression for the group, which may be null
  @param fieldAccumulators zero or more field accumulator pairs
  @return the $group pipeline stage
  @mongodb.driver.manual reference/operator/aggregation/group/ $group
  @mongodb.driver.manual meta/aggregation-quick-reference/#aggregation-expressions Expressions