Make `.elapsed` reflect the entire response time.

[lovelydinosaur]

I think we should probably switch the behavior of .elapsed so that rather than reflecting the "time for the response to start", it is instead only available once .close() has been called, and is calculated from the start of the request to the point that the response is closed.

That way it'll actually properly reflect the time taken for the entire request/response cycle.

[lovelydinosaur]

Two different options here during streaming responses...

• .elapsed is not available until the response is closed.
• .elapsed is available, but will reflect "elapsed so far", until the response is finally closed.

I guess we probably just want to go with the former, as a more constrained option.

[lovelydinosaur]

We'll also want to note in the requests compat that our .elapsed property is different to requests. (Which I assume uses time-to-start-response?)