Get Varnish stats with VMOD dyncounters

[AbdelrahmanElawady]

Get Varnish stats by using VMOD dyncounters to create custom stats and fetching them from vstats.

Closes: #7782

---

Which Traffic Control components are affected by this PR?

• Traffic Monitor
• CDN in a Box

What is the best way to verify this PR?

• Run CIAB with Varnish
• Query some valid endpoints for 2xx like: docker-compose exec enroller curl -L "http://video.demo1.mycdn.ciab.test"
• Query invalid endpoints for 4xx like: docker-compose exec enroller curl -L "http://video.demo1.mycdn.ciab.test/any"
• Either query Traffic Monitor endpoint /publish/DsStats?stats=status_4xx,status_2xx to see the counters increasing or navigate to Traffic Portal UI for similar results.

PR submission checklist

• [x] This PR has tests
• [x] This PR has documentation
• [ ] This PR has a CHANGELOG.md entry
• [x] This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[codecov[bot]]

Codecov Report

Merging #7805 (8d22863) into master (a2720ee) will decrease coverage by 2.90%. Report is 19 commits behind head on master. The diff coverage is 69.84%.

@@             Coverage Diff              @@
##             master    #7805      +/-   ##
============================================
- Coverage     31.77%   28.87%   -2.90%     
  Complexity       98       98              
============================================
  Files           717      601     -116     
  Lines         82538    77237    -5301     
  Branches        965       90     -875     
============================================
- Hits          26224    22303    -3921     
+ Misses        54155    52843    -1312     
+ Partials       2159     2091      -68     

Flag	Coverage Δ
golib_unit	53.54% <0.00%> (-0.02%)	:arrow_down:
grove_unit	12.02% <ø> (ø)
t3c_unit	5.91% <ø> (ø)
traffic_monitor_unit	26.93% <77.19%> (+0.48%)	:arrow_up:
traffic_ops_unit	21.50% <ø> (+0.04%)	:arrow_up:
traffic_portal_v2	?
traffic_stats_unit	10.78% <ø> (ø)
unit_tests	25.69% <69.84%> (-3.39%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
lib/varnishcfg/vclbuilder.go	7.84% <0.00%> (-0.33%)	:arrow_down:
lib/varnishcfg/stats.go	0.00% <0.00%> (ø)
traffic_monitor/cache/vstats.go	70.11% <77.19%> (+18.50%)	:arrow_up:

... and 120 files with indirect coverage changes

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