fix: send trace origin correctly

[lcian]

Description

In #858 we added reporting for trace origin.
The implementation there was actually incorrect because we attached origin to the trace data.
Instead, we need to add a new field on TraceContext called origin, and write it there.
Technically this is a breaking change, but I imagine it's very unlikely that a user needs to construct this struct.

Issues

Closes #907
Closes RUST-110

[codecov]

Codecov Report

❌ Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.46%. Comparing base (75a8c03) to head (064c8ce).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #906      +/-   ##
==========================================
+ Coverage   73.44%   73.46%   +0.01%     
==========================================
  Files          64       64              
  Lines        7502     7506       +4     
==========================================
+ Hits         5510     5514       +4     
  Misses       1992     1992              

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[linear]

RUST-110 Trace origin is sent incorrectly