Emit the status code for responses, in addition to other stats

[elliottt]

Emit the response status code when emitting stats about the request that was handled. Additionally, move the tracing span so that it covers all messages emitted for request handling, not just run_guest.

Here's a sample of what the new output looks like (when -v is passed):

2024-08-14T22:41:59.706626Z  INFO Listening on http://127.0.0.1:7676
2024-08-14T22:42:03.128146Z  INFO request{id=0}: handling request GET http://localhost:7676/
2024-08-14T22:42:03.128951Z  INFO request{id=0}: request completed using 1.1 MB of WebAssembly heap
2024-08-14T22:42:03.128999Z  INFO request{id=0}: request completed in 756µs
2024-08-14T22:42:03.129141Z  INFO request{id=0}: response status: 200

[acw]

that's helpful!