Can protobuf service method return primitive type?

You can return scalar datatypes like bool, int, etc by making use of wrappers.proto

service.proto file:

import "message.proto";
import "google/protobuf/wrappers.proto";

service Service {
    rpc request (Request) returns (.google.protobuf.BoolValue); 
}

If you want to return a primitive type, wrap it in a message and return it:

message Name {
  string name = 1;
}

In case you don't want to return anything, void I mean, you can just create an empty message:

message Void {} 

message Name {
  string name = 1;
}

..
service MyService{
  rpc MyFunc(Name) returns (Void);
}

No, you cannot use a primitive type as either the request or response. You must use a message type.

This is important because a message type can be extended later, in case you decide you want to add a new parameter or return some additional data.