One-line feature description (can be used as a release note):

Improvements allowing Horizontal Pod Autoscaler to reach proper size faster

• Primary contact (assignee): @jbartosik
• Responsible SIGs: @kubernetes/sig-autoscaling
• Design proposal link (community repo):
• Link to e2e and/or unit tests:
  • Unit tests: https://github.com/kubernetes/kubernetes/tree/master/pkg/controller/podautoscaler, https://github.com/kubernetes/kubernetes/blob/master/test/e2e/autoscaling/custom_metrics_stackdriver_autoscaling.go
• Reviewer(s): @DirectXMan12 @mwielgus @MaciekPytel
• Approver (likely from SIG/area to which feature belongs): @DirectXMan12 @mwielgus
• Feature target (which target equals to which milestone):
  • Alpha release target (x.y) 1.12
  • Beta release target (x.y) 1.12
  • Stable release target (x.y)