Extending Protobuf Messages

This is not the exact answer to your question but we can do something like this to share common parameters.

message Child1 { 
    required int c1 = 2;
    required string c2 = 3;
}

message Child2 { 
    required int c1 = 2;
    required string c2 = 3;
}

message Request {
    required string common1 = 0;
    optional string common2 = 1;
    oneof msg { Child1 c1 = 2; Child2 c2 = 3; }

}

Other option is to use extend keyword

message Parent {
    required string common1 = 0;
    optional string common2 = 1;
}

message Child1 { 
    extend Parent
    {       
        optional Child1 c1 = 100;
    }

    required int c1 = 2;
    required string c2 = 3;
}