public static GroupOperation output(Aggregator... aggrs) {
        return new GroupOperation("output", aggrs);
    }
Helper class for generating group syntax
https://docs.vespa.ai/en/reference/grouping-syntax.html

basically the syntax is exactly the same as Vespa group syntax.
The only exception "max" in the Vespa group syntax which represents 'max returned documents',
is replaced by "maxRtn" in the dsl lib.
client/src/main/java/ai/vespa/client/dsl/G.java