How to retrieve value of NumericDocValuesField in similarity

[Ross Woolf]

The JavaDocs for NumericDocValuesField indicates that this field value can
be used for scoring.  The example shows how to store the field, but I am
unclear as to how to retrieve the value of the field while in a similarity
to use it when scoring a document?  Can someone point me to an example or
give me one that demonstrates how I can fetch the value associated with the
document being scored while in a similarity class that I have created?