[v0.16.0] Allow use of PodAutoscalerUseRestClient without addon metrics-server.

[dominicgunn]

Changes

• Allows us to enable --horizontal-pod-autoscaler-use-rest-clients without requiring us to use the metrics-server addon.

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[codecov-commenter]

Codecov Report

Merging #1860 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1860   +/-   ##
=======================================
  Coverage   24.40%   24.40%           
=======================================
  Files          98       98           
  Lines        5117     5117           
=======================================
  Hits         1249     1249           
  Misses       3728     3728           
  Partials      140      140           

Impacted Files	Coverage Δ
pkg/api/types.go	0.00% <ø> (ø)

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[dominicgunn]

/lgtm

[k8s-ci-robot]

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/lgtm

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