Minimize memory use for un-sampled spans

[MrAlias]

If we call start_tracking_span it should make sure the context is propagated. That means we need to call stop_tracking_span when the span in ended. However, we use the active_spans_by_span_ptr to fetch the span in the end probe so not updating the map means we won't be able to fetch the span in the return probe, and won't clean the span context maps properly.

I'd prefer to save maps space for un-sampled spans but it seems more tricky than I initially thought.

Originally posted by @RonFed in #1107 (comment)