Use the new helper from pkg for mem stats

[vagababov]

Fixes #9787.

Use the new helper in the pkg

/assign @julz mattmoor

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[vagababov]

/hold
For actual update of pkg.

[codecov]

Codecov Report

Merging #9788 into master will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #9788      +/-   ##
==========================================
- Coverage   87.77%   87.73%   -0.04%     
==========================================
  Files         180      180              
  Lines        8521     8551      +30     
==========================================
+ Hits         7479     7502      +23     
- Misses        792      797       +5     
- Partials      250      252       +2     

Impacted Files	Coverage Δ
cmd/activator/main.go	0.00% <0.00%> (ø)
cmd/autoscaler/main.go	13.13% <ø> (-0.14%)	⬇️
cmd/queue/main.go	0.00% <0.00%> (ø)
pkg/autoscaler/statforwarder/forwarder.go	81.18% <0.00%> (-1.96%)	⬇️
pkg/autoscaler/bucket/bucket.go	100.00% <0.00%> (ø)

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[vagababov]

/hold cancel

This is ready

[yanweiguo]

/lgtm