Currently, the `serve` function spawns tasks `instrument`ed with a
`tracing` span for each accepted connection. This span is at the `DEBUG`
level, so it's disabled with the default tracing configuration. This is
fine, as the per-connection context is relatively verbose and is mainly
useful for debugging.

However, we also rely on this span for propagating the `INFO`-level
spans which indicate which part of the proxy an event occurred in
(inbound, outbound, etc). When the `DEBUG` span is enabled, it will be a
child of these spans, so they are propagated to the spawned tasks.
However, when the `DEBUG` span is _not_ enabled, nothing propagates the
`INFO` spans. Since the default `tracing` configuration enables `INFO`
but not `DEBUG`, we want those spans to be propagated to the tasks
spawned in `serve`.

This commit fixes the missing spans by moving the spawn inside of the
`Span::in_scope` call, and using `in_current_span` rather than
`instrument`. Now, if the per-connection `DEBUG` span is enabled, it
will be the current span...but if it isn't, the `INFO` span will still
be current, so the task will still have the `INFO` span as part of its
span context regardless.

Alternatively, we could have fixed this by changing the `instrument()`
call to:
```rust
    .instrument(span)
    .in_current_span()
```
so that the task is always spawned in both the `DEBUG` span _and_ the
current span. However, this is a bit less efficient, as it wraps the
tasks in both spans even when the `INFO` span is not needed, so every
time the task is polled, we would enter both spans.