public static <K> GuavaGroupByBuilder<K> groupBy(Expression<K> key) {
        return new GuavaGroupByBuilder<K>(key);
    }
Create a new GroupByBuilder for the given key expression

@param key key for aggregation
@return builder for further specification
querydsl-guava/src/main/java/com/querydsl/core/group/guava/GuavaGroupBy.java