sentry-core: make TraceContext publicly readable

[tommilligan]

Raised in review of #533 (comment)

This PR exposes the TraceContext stored on a Transaction or Span.

All data on the TraceContext is publicly readable in other SDKs, such as Python or Javascript.

• Rust struct:

  sentry-rust/sentry-types/src/protocol/v7.rs

  Line 1439 in 5013eb1

  pub struct TraceContext {

• Python analagous class: https://github.com/getsentry/sentry-python/blob/d0eed0ee828684f22fe2a2b28b02cf7f4ce8c74a/sentry_sdk/tracing.py#L81

[codecov-commenter]

Codecov Report

Merging #534 (e65304d) into master (abc2b34) will decrease coverage by 0.13%.
The diff coverage is 0.00%.

❗ Current head e65304d differs from pull request most recent head a15b835. Consider uploading reports for the commit a15b835 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #534      +/-   ##
==========================================
- Coverage   69.66%   69.53%   -0.14%     
==========================================
  Files          66       66              
  Lines        6564     6577      +13     
==========================================
  Hits         4573     4573              
- Misses       1991     2004      +13     

[Swatinem]

its a bit unfortunate this is behind a lock so you need to clone :-(

but oh well

[tommilligan]

its a bit unfortunate this is behind a lock so you need to clone :-(

but oh well

Agreed. I think in future there would be scope for handling this better with something like parking_lot's MutexGuard::map, if you're open to changing the implementation of Mutex: https://docs.rs/lock_api/latest/lock_api/struct.MutexGuard.html#method.map

But I think for this case, a one-off clone is not too bad.

[Swatinem]

I would be hesitant to give out more lock guards, with the recent deadlock I tracked down and fixed quite painfully: #536

We should rather remove the need for locking in the first place somehow, though I have no good ideas at the moment.