According to the documentation, proto files must be placed in the src/main/proto directory, which is what I had missed. Here is the current structure of my datastoreproto module:

core/
└── datastoreproto/
    ├── build/
    ├── src/
    │   ├── main/
    │   │   └── proto/
    │   │       └── auth_info.proto
    │   └── AndroidManifest.xml
    ├── test/
    ├── .gitignore
    ├── build.gradle.kts
    ├── consumer-rules.pro
    └── proguard-rules.pro