This was a bit harder to identify: during an `io_loop` run we suspend
the current span before handing over to `io_loop`, and later when a callback
is called we resume the span again. Depending on how we return from
the `io_loop` instance that is used to drive the startup, we either
have resumed the last span, or we don't. Since we start a span before
`io_loop` and want it to be emitted afterwards, we need to take care
of the case where we returned from a callback that did not resume, and
therefore the current context is empty.
Making `trace_span_resume` idempotent means we can just resume it
manually.
Ideally we'd push the suspend / resume logic down into `io_loop`
itself, and then we'd have just one place. Maybe suspend and resume
callbacks that can be configured in `io_loop`?
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