@Override
    protected void onMerge(AggregationResult result) {
        sum.add(((AverageAggregationResult)result).sum);
        count += ((AverageAggregationResult)result).count;
    }
Returns the average value of the results. Because the result can be any numeric type, this method returns a
{@link NumericResultNode} object.

@return The average result value.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/AverageAggregationResult.java