The metering sensor is under the mirror and is used to set aperture and shutter speed. This means light is getting to the metering sensor. Obviously, if it gets to the metering sensor, then it probably gets to the main sensor as well, its just not very much light.
The metering sensor was under the mirror in old designs, but these days it's actually up in the pentaprism housing (labeled in the upper right of this diagram taken from a Canon tech report):
So, light that reaches the metering sensor should be blocked from hitting the image sensor by the mirror when it's flipped up during the exposure.
100% confirmed - for my 5D3. I have done these crazy tests and most importantly - every frame I exposed (using manual focus) came out completely black. Therefore, no stray light from the “metering leakage” is getting to the sensor. I even did a 120 second bulb exposure with direct sunlight onto the LCD panel at ISO800 and only the slightest stray light was witnessed and that was found to come through the VF cover – so it isn’t perfect either. When I removed this tiny VF leak, a 256 second bulb exposure got absolutely nothing. To put in context, a proper exposure was 1/1000 second at f/8.