pub fn protoc_arg<A: AsRef<str>>(mut self, arg: A) -> Self {
        self.protoc_args.push(arg.as_ref().into());
        self
    }
Configure Prost `protoc_args` build arguments.

Note: Enabling `--experimental_allow_proto3_optional` requires protobuf >= 3.12.
tonic-build/src/prost.rs