@Override
    protected void onSerialize(Serializer buf) {
        super.onSerialize(buf);
        buf.putInt(null, values.size());
        for (int value : values) {
            buf.putInt(null, value);
        }
    }
Serializes the Sketch.

Serialization format
==================
Number of elements:      4 bytes
Elements:            N * 4 bytes
@param buf Serializer
searchlib/src/main/java/com/yahoo/searchlib/aggregation/hll/SparseSketch.java