fix(sentry-tracing): switch sentry spans on enter and exit

[saiintbrisson]

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I've been facing a bug with Sentry Trace where concurrent spans end up wrongly nested. At first, I thought this was some strange autogrouping bug and event sent a feedback (sorry Sentry support member 😅).

After digging a little bit, I discovered that the current tracing layer implementation sets the sentry span as active as soon as a tracing span is created, which should not be the case, as spans are only active once entered. Following the same logic, it also reverts to the parent span only when a span is closed, and not when it's exited.

A span can be entered and exited multiple times, for example, when a tracing Instrumented future polls and returns Pending. In case of concurrent Instrumented futures, the current impl ends up nesting Futures under the first created one.

sentry-tracing layer (wrong)

tracing-opentelemetry layer (correct)

I'd say that the best idea would be to not use sentry_core::configure_scope(|s| s.get_span()); when creating a new span to get the parent, and get it from the span extensions instead, but the current behavior might be relied upon already by someone.