Support streaming tail workers over rpc

[danlapid]

As it stands workers tailed by streaming tail workers have to know they are tailing to a streaming tail worker.
This is a limitation that should be refactored away in the future but for the moment we can't depend on hasHandler to infer this over rpc.

At the moment it produces the following error on the traced workerd instance:

exception = workerd/io/trace-stream.c++:976: disconnected: Streaming tail session canceled

This is a known issue preexisting that Felix is working on in a separate PR.

To test use the following files:
config.capnp:

using Workerd = import "/workerd/workerd.capnp";

const tailWorkerExample :Workerd.Config = (
  services = [
    (name = "main", worker = .helloWorld),
    (name = "log", external = ( address = "127.0.0.1:8081", http = ( capnpConnectHost = "capnp" ))),
  ],
  sockets = [ ( name = "http", address = "*:8080", http = (), service = "main" ) ],
);

const helloWorld :Workerd.Worker = (
  modules = [
    (name = "worker", esModule = embed "worker.js")
  ],
  compatibilityDate = "2024-10-14",
  streamingTails = ["log"],
  bindings = [(name = "log", service = "log")]
);

log.capnp:

using Workerd = import "/workerd/workerd.capnp";

const tailWorkerExample :Workerd.Config = (
  services = [
    (name = "log", worker = .logWorker),
  ],
  sockets = [ ( name = "rpc", address = "*:8081", http = ( capnpConnectHost = "capnp" ), service = "log" ) ],
);

const logWorker :Workerd.Worker = (
  modules = [
    (name = "worker", esModule = embed "tail.js")
  ],
  compatibilityDate = "2024-10-14",
);

tail.js:

export default {
  tailStream(...args) {
    console.log('tail in worker b');
    console.log(...args);
    return (...args) => {
      console.log(...args);
    };
  },
};

worker.js:

export default {
  async fetch(req, env) {
    console.log('log from worker a');
    return new Response('response from worker a');
  },
};

bash:

workerd serve deps/workerd/samples/tail-workers/log.capnp
workerd serve deps/workerd/samples/tail-workers/config.capnp
curl http://localhost:8080

[fhanau]

If we're adding streamingTails, samples/tail-workers/config.capnp and src/workerd/api/tail-worker-test.wd-test need to be updated accordingly.

[jasnell]

The basic change here LGTM but @fhanau is correct that the test will need to be updated... as likely will the sample.

[danlapid]

@fhanau @jasnell done, stamps appreciated.