HTTP server exchange observations have incorrect UNKNOWN status tag if the client disconnected

[bclozel]

As seen in #31365, our HTTP server observations mark the response status tag as "UNKNOWN" if the client has disconnected. We could instead refine the behavior there and only use this status if the response has not been committed.

• if the response has been committed, we known that this status was intended
• if the response was not committed when the exchange was cancelled, we're not sure that the current status was indeed the intended one. Depending on the underlying server, there might be a default value set at that point.