memoize balancer for a request

[ElvinEfendi]

What this PR does / why we need it:

This is to make sure for a given request context get_balancer always returns the same balancer.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #4364

Special notes for your reviewer:

[ElvinEfendi]

/test pull-ingress-nginx-test

[ElvinEfendi]

/test pull-ingress-nginx-test

[codecov-io]

Codecov Report

Merging #4365 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #4365      +/-   ##
=========================================
+ Coverage   58.47%   58.5%   +0.03%     
=========================================
  Files          87      87              
  Lines        6528    6528              
=========================================
+ Hits         3817    3819       +2     
+ Misses       2282    2281       -1     
+ Partials      429     428       -1

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

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[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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