Add trace exporting to mt broker

[benmoss]

The MT Broker controller wasn't exporting traces, so downstream components would get trace span parents that never show up in your tracing system.

Proposed Changes

• 🐛 Export traces generated by the mtbroker controller

Release Note

Traces are exported by the mtbroker controller

Before:

After:

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-eventing-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/broker/controller.go	88.9%	85.0%	-3.9

[codecov]

Codecov Report

Merging #5634 (645ae63) into main (0c41e6d) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #5634      +/-   ##
==========================================
- Coverage   82.77%   82.73%   -0.05%     
==========================================
  Files         200      200              
  Lines        6258     6266       +8     
==========================================
+ Hits         5180     5184       +4     
- Misses        748      750       +2     
- Partials      330      332       +2     

Impacted Files	Coverage Δ
pkg/reconciler/broker/controller.go	88.88% <0.00%> (-4.14%)	⬇️
cmd/event_display/main.go	57.89% <0.00%> (+4.04%)	⬆️

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[lionelvillard]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: benmoss, lionelvillard

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