public static FieldUpdate createMultiply(Field field, FieldValue key, Number factor) {
        return new FieldUpdate(field, ValueUpdate.createMultiply(key, factor));
    }
Creates a new field update, with one encapsulated value update that multiplies a weight in a weighted set.

@param field     the field to multiply one of the weights of
@param key the key whose weight in the weighted set to multiply
@param factor the number to multiply by
@return a FieldUpdate specifying the multiplication
@throws IllegalArgumentException if key is not equal to the nested type of the weighted set
@see ValueUpdate#createMultiply(Number)
@see ValueUpdate#createMap(FieldValue, ValueUpdate)
document/src/main/java/com/yahoo/document/update/FieldUpdate.java