[release-1.22] Preserve CloudEvent trace context

[knative-prow-robot]

This is an automated cherry-pick of #794

🐛 Preserve producer-supplied CloudEvents trace context when events pass through the native NATS Broker ingress.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.50%. Comparing base (cfd4d0a) to head (d7e1583).
⚠️ Report is 1 commits behind head on release-1.22.

Files with missing lines	Patch %	Lines
pkg/tracing/tracing.go	83.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-1.22     #795      +/-   ##
================================================
- Coverage         43.55%   43.50%   -0.06%     
================================================
  Files                68       68              
  Lines              4254     4259       +5     
================================================
  Hits               1853     1853              
- Misses             2277     2282       +5     
  Partials            124      124              

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[astelmashenko]

/lgtm
/approve

[knative-prow]

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