what’s the right way to do polymorphism with protocol buffers?

In proto3 the extend keyword has been replaced.
From the docs: If you are already familiar with proto2 syntax, the Any type replaces extensions.

syntax = "proto3";

import "google/protobuf/any.proto";

message Foo {
  google.protobuf.Any bar = 1;
}

But beware: Any is essentially a bytes blob. Most of the times it is better to use Oneof:

syntax = "proto3";

message A {
    string a = 1;
}

message B {
    string b = 1;
}

message Foo {
  oneof bar {
    A a = 1;
    B b = 2;
  }
}