fix: Duration including background task execution time

[yourboirusty]

What does this do?

1. Tracks response end time in the send wrapper rather than measuring time in the finally after full app execution.
2. Minor middleware refactor.

Why do we need it?

1. To stop background task execution time from inflating response time metrics in most cases.
2. Cyclomatic complexity was too large for pre-commit to pass. Took the liberty of refactoring a bit more than absolutely necessary.

Who is this for?

People who use relatively long-running background tasks but still want accurate response times for every endpoint.

Linked issues

Fixes #275

Reviewer notes

If the refactors are too heavy-handed I can tune it back, but it made sense to me to flatten out the low-hanging fruit while I was at it.

This does not cover two potential issues:

• in progress tracker will still only decrement after background tasks have finished,
• requests without response body will still count background tasks into their duration.

I thought it wouldn't cover responses without body, but tests disagree with me.

[marrrcin]

@trallnag any plans to merge this?