want to derive `counters::Count` for structs when they have `count(children)`able enum fields

See this comment I left on #2668 after I saw @jamesmunns hand-implementing Count for a struct because it was a struct:

Oh, huh, I think this is necessary because the counters crate doesn't support deriving Count for a struct, even if it has count(Children) on a field that implements Count? It could be worth adding that to better support cases like this (not in this PR, I'll go make a ticket).

Originally posted by @hawkw in https://github.com/oxidecomputer/hubris/pull/2668#discussion_r3938035867

At a first glance this didn't really seem worth having a derive that worked on structs because what would you count about them, but sometimes they have child fields that are countable, and it's nicer to not have to hand write that.

Start by tracing the counters::Count derive implementation and how it handles count(Children) on enum fields. Compare that behavior with the hand-written Count implementation discussed in pull request #2668. Done means structs containing countable child fields can derive Count without a manual implementation.