Back in the 1980s, Craig Anderton wrote a book called Home Recording for Musicians in which he observed that a recording studio isn't a capturer of reality, it's a processor of reality. You should think of your camera the same way. The light making up an image enters the lens, which changes it, reaches the sensor, which changes it, is passed through various bits of analog circuitry, which change it, and finally becomes digital data by way of an analog-to-digital converter (ADC) which, as you might have guessed, changes it.
Any change is an alteration, and it might be reasonable to call alteration done with a purpose processing. There are undoubtedly some things done on purpose in the analog stages of your camera. ISO adjustment is the elephant in that room because analog amplification is necessary to get the most out of the ADC.
You're not getting information straight from the sensor because it's an analog signal, and analog signals are notoriously difficult to store and reproduce well. Whether you call what happens between there and when you have digital information alteration or processing is debating the trivial. If you're going to split those hairs, the answer is that it's probably some of both.
The ideal place for capturing data for raw files is the output of the ADC, because anything that happens to it beyond that is algorithmic and can be done off-camera. If you find a better way to do something (demosaicing, application of curves, etc.), it's much more economical to update a raw converter that runs on a computer than it is to create, test and distribute new camera firmware. Providing raw data is also something of an acknowledgment that the decisions made by the manufacturer aren't going to please everyone. Doing as little as possible to the data post-ADC gives the photographer the same flexibility he'd have had in a darkroom.
Whether or not the contents of a raw file is straight from the ADC or somehow adulterated is known only to the manufacturers. One implies that it doesn't, and my own inclination is to believe that to be true for reasons that go beyond the scope of answering this question.
As Michael Clark points out in his answer, everything in a raw file that isn't image data is bonus information that can be used in making decisions about how to process it.