feat(tracing): Add method for accessing span data

[loewenheim]

This adds a data method to performance::Span that gives exclusive access to the underlying protocol::Span's data field (a map from String to protocol::Value). Since the inner span is behind a Mutex, the data method returns a Data smart pointer that holds the lock.

[codecov]

Codecov Report

Merging #548 (a12490b) into master (37be013) will decrease coverage by 0.10%.
The diff coverage is 0.00%.

❗ Current head a12490b differs from pull request most recent head 3dfbc48. Consider uploading reports for the commit 3dfbc48 to get more accurate results

Additional details and impacted files

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- Coverage   70.68%   70.59%   -0.10%     
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  Files          66       66              
  Lines        6649     6658       +9     
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  Hits         4700     4700              
- Misses       1949     1958       +9     

[Swatinem]

I’m not entirely sure if we call out locking explicitly in other parts of the SDK, but it might be good to at least mention it, as otherwise this is a footgun that could lead to deadlocks.

[loewenheim]

Yeah, I was wondering about this as well. I'll add a note.

[Swatinem]

Another idea would be to take a FnOnce, but that is also not quite ergonomic.