fix: Make metered channel accounting take unused dropped Permits into account

[huitseeker]

Context

We introduced metered channels in #682. Then we started using mpsc::Permit in #724 to implement composable backpressure.

The issue

Permits increase the occupancy metric of their underlying channel when acquired. They decrease that occupancy when they are used to send an element that's received. They do not decrease this occupancy when the permit is dropped without being utilized to send anything.

This leads to the following graphs on precisely the three channels which in #724 were made to use permits:

For context, those channels have a bounded capacity of 1000 elements (due to the underlying use of a tokio::mpsc::channel). They show occupancy far above that, which constitutes the bug.

The fix

We first introduce a (failing) test test_reserve_and_drop that shows the issue, and fix it by making permits from metered channels decrease the occupancy of their underlying channel when they are dropped without having sent an element.