use nkeys for counting lua table elements

[ElvinEfendi]

What this PR does / why we need it:
This new API function introduced in openresty/luajit2@0e646b5 can be JIT compiled.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

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[codecov-io]

Codecov Report

Merging #3803 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3803      +/-   ##
==========================================
+ Coverage   57.76%   57.79%   +0.03%     
==========================================
  Files          87       87              
  Lines        6459     6459              
==========================================
+ Hits         3731     3733       +2     
+ Misses       2296     2295       -1     
+ Partials      432      431       -1

Impacted Files	Coverage Δ
internal/ingress/metric/collectors/process.go	90.42% <0%> (+2.12%)	⬆️

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[aledbf]

@ElvinEfendi can we merge this change?

[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, ElvinEfendi

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