public GroupingNode setLabel(String str) {
        label = str;
        return this;
    }
Assigns a label to this grouping expression. The label is applied to the results of this expression so that they
can be identified by the caller when processing the output.

@param str The label to assign to this.
@return This, to allow chaining.
container-search/src/main/java/com/yahoo/search/grouping/request/GroupingNode.java