@Override
    protected void onSerialize(Serializer buf) {
        super.onSerialize(buf);
        buf.putLong(null, count);
        serializeOptional(buf, sum);
    }
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