Focal length affects the field of view. Spot metering which is around 3-6 percent of the screen will cover bigger area of a scene/object in wide angle than a telephoto where everything is magnified.On a second thought, I realized that the lens shouldn't effect the metering. Since DSLR in camera meter is caliberated to read 18% grey, any change in luminance of light would only alter the values around 18% grey. The scene might look a bit dark or a bit light but since the camera meter is designed to read only the pre defined values around 18% gray, any lens should not effect the metering.
Henry, why should focal length change the metering, especially if you spot meter?