[release 1.8] Cherry Pick #2864

[mattmoor]

git cherry-pick 413fab783d9a92b660ba968e5b35ded8756be004

[mattmoor]

commit 413fab783d9a92b660ba968e5b35ded8756be004 (upstream/main, main)
Author: Matt Moore <mattmoor@vmware.com>
Date:   Tue Sep 1 16:54:00 2020 -0700

    Replace uncached Get to fix knative-sandbox/net-contour#226 (#2864)
    
    Replace the dynamic client Get (uncached) with a read from
    the informer's lister cache.  This code path is evaluated
    for every HTTP Proxy on each DAG rebuild, and the call to
    the API Server on each results in a thundering herd that
    triggers the built-in rate limiting in k8s.io/client-go.
    
    With this change, the only API Server calls on DAG rebuilds
    happen only when our cached copy of the HTTP Proxy shows
    that we need to perform a status update.
    
    Updates #2857
    
    Signed-off-by: Matt Moore <mattmoor@vmware.com>

[codecov]

Codecov Report

Merging #2865 into release-1.8 will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@               Coverage Diff               @@
##           release-1.8    #2865      +/-   ##
===============================================
- Coverage        76.48%   76.45%   -0.04%     
===============================================
  Files               73       73              
  Lines             5712     5712              
===============================================
- Hits              4369     4367       -2     
- Misses            1258     1259       +1     
- Partials            85       86       +1     

Impacted Files	Coverage Δ
cmd/contour/serve.go	2.52% <0.00%> (-0.01%)	⬇️
internal/k8s/statusupdater.go	20.61% <0.00%> (+0.21%)	⬆️
internal/dag/cache.go	96.86% <0.00%> (-0.79%)	⬇️

[youngnick]

LGTM