The wonderful word Auftragsdatenverarbeitung describes the scenario when a service contractor receives personal data to process it. The principal is responsible for the contractor to adhere to legal regulations. Adherence has to be monitored throughout contract duration and we want to remember the obligation to document all that.
This is why Google Analytics provides a Contract for Auftragsdatenverarbeitung (PDF) to German users. It is recommended to have that contract signed if you want to use Google Analytics in Germany. Terrible things will happen otherwise.