My bad! "NumericalValue" gives you the metric value. Try "Metric" instead! And you should also be able to use an absolute specifier. Possible values are:
<xs:simpleType name="TableColumnContentType">
<xs:restriction base="xs:string">
<xs:enumeration value="Host"/>
<xs:enumeration value="Process"/>
<xs:enumeration value="Agent"/>
<xs:enumeration value="NumericalValue"/>
<xs:enumeration value="AlertValue"/>
<xs:enumeration value="MinValue"/>
<xs:enumeration value="MaxValue"/>
<xs:enumeration value="DataPointCount"/>
<xs:enumeration value="StringValue"/>
<xs:enumeration value="LastPrefixSegment"/>
<xs:enumeration value="PrefixPattern"/>
<xs:enumeration value="Metric"/>
<xs:enumeration value="Timestamp"/>
<xs:enumeration value="Description"/>
<xs:enumeration value="Parameter"/>
</xs:restriction>
</xs:simpleType>