> they could do this in a way that they are worthless for potential competitors (e.g. if the contribution depends on a proprietary Amazon system)
That is not allowed.
"""The "Corresponding Source" for a work in object code form means all the source code needed to generate, install, and (for an executable work) run the object code and to modify the work, including scripts to control those activities. However, it does not include the work's System Libraries, or general-purpose tools or generally available free programs which are used unmodified in performing those activities but which are not part of the work. For example, Corresponding Source includes interface definition files associated with source files for the work, and the source code for shared libraries and dynamically linked subprograms that the work is specifically designed to require, such as by intimate data communication or control flow between those subprograms and other parts of the work."""
(Emphasis on that last part mine.)
"""Notwithstanding any other provision of this License, if you modify the Program, your modified version must prominently offer all users interacting with it remotely through a computer network (if your version supports such interaction) an opportunity to receive the Corresponding Source of your version by providing access to the Corresponding Source from a network server at no charge, through some standard or customary means of facilitating copying of software."""
If Amazon modifies Grafana so it depends on a proprietary Amazon system, the source code for that system is included in the Corresponding Source, and if they allow users to interact with that modified version, they need to provide them with access to the proprietary system's source code.