Properly shutdown tracer when the work is done

[mgencur]

This is a follow-up on knative/pkg#2566

Proposed Changes

• The shutdown function will flush any buffered tracing Spans

Release Note

[codecov]

Codecov Report

Merging #13226 (1fa6a44) into main (dbd3fc3) will decrease coverage by 0.06%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main   #13226      +/-   ##
==========================================
- Coverage   86.67%   86.60%   -0.07%     
==========================================
  Files         196      196              
  Lines       14493    14495       +2     
==========================================
- Hits        12562    12554       -8     
- Misses       1634     1642       +8     
- Partials      297      299       +2     

Impacted Files	Coverage Δ
cmd/activator/main.go	0.00% <0.00%> (ø)
pkg/queue/sharedmain/main.go	0.63% <0.00%> (-0.01%)	⬇️
pkg/reconciler/revision/background.go	90.90% <0.00%> (-1.82%)	⬇️
pkg/reconciler/configuration/configuration.go	83.67% <0.00%> (-1.54%)	⬇️
pkg/activator/net/revision_backends.go	92.19% <0.00%> (-0.58%)	⬇️

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[dprotaso]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, mgencur

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• cmd/activator/OWNERS [dprotaso]
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