end-of-request USDT probes, log entries don't always happen

[davepacheco]

After #701, there are potentially two async tasks involved in request handlers:

• the one in which hyper invokes us when a new request comes in. This one also runs the request handler if using HandlerTaskMode::CancelOnDisconnect.
• if using HandlerTaskMode::Detached, the one in which the request handler runs

If the request is cancelled (e.g., if the client disconnects), the first of these tasks gets cancelled. With HandlerTaskMode::Detached, the request handler still runs to completion. Unfortunately, the "request completed" log entry USDT probes happen in the first of these contexts. So you don't get these events when the client request was cancelled. This makes it much harder to write correct DTrace scripts and also makes it much harder to debug from logs since you can't assume there's a "request completed" log entry for all requests that did essentially complete.

(This was noticed while debugging oxidecomputer/omicron#5111.)