drop use of knative.dev/pkg metricskey in tests

[dprotaso]

Follow up on: #11658

missed the keys use in the tests

[knative-prow-robot]

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[codecov]

Codecov Report

Merging #11679 (2be2285) into main (8076b8b) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #11679      +/-   ##
==========================================
+ Coverage   87.81%   87.85%   +0.04%     
==========================================
  Files         190      190              
  Lines        9249     9249              
==========================================
+ Hits         8122     8126       +4     
+ Misses        876      875       -1     
+ Partials      251      248       -3     

Impacted Files	Coverage Δ
pkg/activator/net/revision_backends.go	91.51% <0.00%> (-0.90%)	⬇️
pkg/reconciler/configuration/configuration.go	86.92% <0.00%> (+0.76%)	⬆️
pkg/autoscaler/statforwarder/processor.go	94.44% <0.00%> (+5.55%)	⬆️
pkg/autoscaler/statforwarder/forwarder.go	96.29% <0.00%> (+5.55%)	⬆️

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[skonto]

/lgtm