Add array of user agents for health check in drainer

[nader-ziada]

Changes

• Add an array of user agent values in drainer that would trigger health check
• context from serving PR: user pkg drain for draining in queue proxy serving#12033, comment: user pkg drain for draining in queue proxy serving#12033 (comment)

/kind enhancement

Fixes #

Release Note

Add HealthcheckUserAgents to drainer to enable generic headers to be used for healthcheck

[nader-ziada]

/cc @dprotaso @julz

[codecov]

Codecov Report

Merging #2331 (2e684b4) into main (95f98e5) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2331      +/-   ##
==========================================
+ Coverage   64.49%   64.56%   +0.07%     
==========================================
  Files         226      226              
  Lines        9663     9668       +5     
==========================================
+ Hits         6232     6242      +10     
+ Misses       3155     3152       -3     
+ Partials      276      274       -2     

Impacted Files	Coverage Δ
network/handlers/drain.go	87.50% <100.00%> (+1.22%)	⬆️
controller/controller.go	86.49% <0.00%> (+0.36%)	⬆️
test/gcs/mock/mock.go	92.47% <0.00%> (+2.15%)	⬆️

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[dprotaso]

just some minor stuff

[dprotaso]

just some bike shedding on the variable name & casing

note since the pkg cut had occurred this change won't be picked up till after the 1.0 release

[nader-ziada]

note since the pkg cut had occurred this change won't be picked up till after the 1.0 release

no urgency since its mostly cleanup, will hold the serving PR until this is released

[dprotaso]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, nader-ziada

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