SKUs: log the return value of top level functions called from C++

[evq]

per https://docs.rs/tracing/latest/tracing/attr.instrument.html, this will log the return value of these functions (INFO aka VLOG(2) on success, WARN aka VLOG(1) on failure.

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Test Plan:

[evq]

hm looks like this failed as we're using tracing 0.1.37, level = , we'll need to upgrade to 0.1.40 to use it ( though apparently added in 0.1.38, it and 0.1.39 were yanked. )

[rillian]

though apparently added in 0.1.38, it and 0.1.39 were yanked.

Probably for use-after-free in Instrument::into_inner (which we don't call).

[rillian]

lgtm

[evq]

going foward with merge as CI failure is due to unrelated npm audit