Reproduction of issue with tail event not serializable

[edmundhung]

Similar to #3706, we are using a "proxy" Worker placed in front of a user Worker to emulate static asset serving locally. However, it seems like tail events cannot be proxied over a service binding because they aren't serializable.

Would it be possible to make TraceItem (or tail events in general) serializable?

To reproduce, run:

pnpm dlx workerd serve --verbose samples/tail-event-proxy/config.capnp
curl http://localhost:8080

You will see an error like this from workerd:

workerd/io/worker.c++:2126: info: uncaught exception; source = Uncaught (in promise); stack = DataCloneError: Could not serialize object of type "TraceItem". This type does not support serialization.
    at Object.tail (worker:3:27)
workerd/io/io-context.c++:377: info: uncaught exception; exception = workerd/jsg/_virtual_includes/iterator/workerd/jsg/value.h:1526: failed: jsg.DOMException(DataCloneError): Could not serialize object of type "TraceItem". This type does not support serialization.

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