The basic idea is we maintain an annotation in the protos specifying minor version and some semantic hash of the proto contents. A bump of the minor version is forced when the hash changes by presubmit tooling. The minor version will be provided in DiscoveryRequests (while the major version is a part of the package/type name itself). See https://docs.google.com/document/d/1xeVvJ6KjFBkNjVspPbY_PwEDHC7XPi0J5p1SqUXcCl8/edit#heading=h.jlxlwbbptdag and https://docs.google.com/document/d/1xeVvJ6KjFBkNjVspPbY_PwEDHC7XPi0J5p1SqUXcCl8/edit#heading=h.9i8j2wwt08lg for further details.

This issue tracks finalizing the design and implementation and updating API_VERSIONING.md to reflect.