feat: add max-connections-per-listener config option

[flawedmatrix]

Adds a setting for max-connections-per-listener to limit the number of active connections to a listener.

fixes #5654

[flawedmatrix]

cc @christianang

[github-actions]

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[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (c1dd2da) 78.81% compared to head (c593eac) 78.82%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6058      +/-   ##
==========================================
+ Coverage   78.81%   78.82%   +0.01%     
==========================================
  Files         138      138              
  Lines       19765    19784      +19     
==========================================
+ Hits        15577    15595      +18     
- Misses       3885     3886       +1     
  Partials      303      303              

Files	Coverage Δ
cmd/contour/servecontext.go	83.92% <100.00%> (+0.03%)	⬆️
internal/xdscache/v3/runtime.go	100.00% <100.00%> (ø)
pkg/config/parameters.go	86.51% <100.00%> (+0.11%)	⬆️
cmd/contour/serve.go	20.75% <0.00%> (-0.03%)	⬇️

... and 2 files with indirect coverage changes

[sunjayBhatia]

breadcrumb to think about when I get a chance to do a full review: think about admin/stats/etc. Listeners

[sunjayBhatia]

breadcrumb to think about when I get a chance to do a full review: think about admin/stats/etc. Listeners

so far my thought is to exclude those Listeners from this change since the shouldn't be at as much risk from a DOS attack (should not be exposed outside of the cluster, or even node), but we could do a follow on to set a reasonable default number of max connections for these listeners (or even make the limit configurable)

If we do want to ever set the global connection limit, that applies to all Listeners including the admin one, stats, etc., so we would have to make sure those don't get starved, and maybe set the "ignore global limit" flag for them

[skriss]

One little changelog nit, otherwise LGTM. I agree with @sunjayBhatia that it should be fine to not apply this setting to the admin/stats listeners.

[flawedmatrix]

Yeah, that makes sense. If we ever need to set a listener connection limit on the admin and stats endpoint that can be a follow-on change.

[skriss]

LGTM, will leave for @sunjayBhatia to review as well. Thanks @flawedmatrix and @christianang!