[release-1.19] drop unnecessary 'kn.activator.proxy' metric/span attribute

[knative-prow-robot]

This is an automated cherry-pick of #16045

[dprotaso]

/lgtm
/approve

[knative-prow]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.23%. Comparing base (435caad) to head (6e335ed).
⚠️ Report is 1 commits behind head on release-1.19.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-1.19   #16047      +/-   ##
================================================
+ Coverage         80.19%   80.23%   +0.03%     
================================================
  Files               214      214              
  Lines             16907    16905       -2     
================================================
+ Hits              13559    13563       +4     
+ Misses             2985     2981       -4     
+ Partials            363      361       -2     

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