Protodump is a utility to dump all Protobuf file descriptors from a given binary as *.proto files:
git clone https://github.com/arkadiyt/protodump
cd protodump
go build -o protodump cmd/main.go
./protodump -file <file to extract from> -output <output directory>
This was thrown together in a day and still has bugs :)
Feel free to contact me on twitter: https://twitter.com/arkadiyt