coverage: Simplify creation of sum counters
A small and self-contained improvement, extracted from some larger changes that I'm still working on.
Ultimately I want to avoid creating these sum counter-expressions in some cases (in favour of just adding physical counters directly to the nodes we care about), so a good incremental move towards that is splitting the “gather edge counters” step out from the ”build a sum of those counters” step.
Creating an extra intermediate vector should have negligible cost (and coverage isn't exercised by the benchmark suite anyway). The removed logging is redundant with the `#[instrument(..)]` logging we already have on the underlying method calls.
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