Add external_propagation_context support

[sl0thentr0py]

If we are on an external tracing system like otel, we allow registering a new source of trace_id/span_id that takes precedence over the scope's propagation_context.

• Also reworked logs and metrics to use get_trace_context
• Cleaned up handling of get_trace_context that is still messy but a bit more clearer now regarding which underlying propagation_context is used

[codecov]

Codecov Report

❌ Patch coverage is 84.37500% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.91%. Comparing base (2397b15) to head (203d4d1).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/client.py	78.57%	0 Missing and 3 partials ⚠️
sentry_sdk/scope.py	88.88%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5051      +/-   ##
==========================================
- Coverage   83.93%   83.91%   -0.03%     
==========================================
  Files         179      179              
  Lines       17887    17894       +7     
  Branches     3179     3177       -2     
==========================================
+ Hits        15013    15015       +2     
- Misses       1906     1908       +2     
- Partials      968      971       +3     

Files with missing lines	Coverage Δ
sentry_sdk/scope.py	88.65% <88.88%> (+0.16%)	⬆️
sentry_sdk/client.py	83.65% <78.57%> (-0.13%)	⬇️

... and 3 files with indirect coverage changes

[sl0thentr0py]

moved these into get_trace_context for consistency

[sl0thentr0py]

propagation_context is actually never None, but this implementation is whack so we need this redundant check...

[sl0thentr0py]

for some future cleanup, if you see get_active_propagation_context that's also really shady. I generally dislike that so many instance methods on this scope call and fetch other scopes themselves, it's entirely spaghetti but somehow works in the common code paths.

[sentrivana]

Looking good as far as I can tell. Thanks for centralizing the logic more, def an improvement to directly accessing the propagation context from random places 😢