public final Serializer serialize(Serializer buf) {
        onSerialize(buf);
        return buf;
    }
Serializes the content (excluding the identifier) of this class into the given byte buffer. If you need the
identifier serialized, use the {@link #serializeWithId(Serializer)} method instead. This method invokes the
{@link #onSerialize(Serializer)} method.

@param buf The buffer to serialize to.
@return The buffer argument, to allow chaining.
vespajlib/src/main/java/com/yahoo/vespa/objects/Identifiable.java