Automatically record request/responses for o11y

In the context of invoking the `RequestRecorder` calls for HTTP requests and responses @westarle notes:

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Nit: I get a mildly worried about matching the on_x_response to the on_x_requests when they are distributed. I think our tests cover things practicality right now, but I never know what complexity we'll add in the future.

A couple ideas:

- in RequestRecorder have on_http_request return a guard that you must_use to call finish(result)
- Have ClientSnapshot do accounting and put a debug assert in drop

_Originally posted by @westarle in https://github.com/googleapis/google-cloud-rust/pull/5176#discussion_r3005644330_

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I think the "One True Answer" is using tower layers... but our `ReqwestClient` is not structured around them.

A different structural answer is to decorate the future that returns the `Result`.

Those are good cleanup opportunities for `ReqwestClient`.