[v0.14.x] 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.

[k8s-ci-robot]

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[codecov-io]

Codecov Report

Merging #1844 into v0.14.x will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           v0.14.x    #1844   +/-   ##
========================================
  Coverage    24.93%   24.93%           
========================================
  Files           98       98           
  Lines         5078     5078           
========================================
  Hits          1266     1266           
  Misses        3673     3673           
  Partials       139      139           

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

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