Horizontal Pod Autoscaler (HPA) fails getting CPU consumption and reports unknown for current cpu usage : "the HPA was unable to compute the replica count" in OCP 4 with apiVersion autoscaling/v1

Issue

• The Horizontal Pod Autoscaler (HPA) metrics shows <unknown> in the summary view under TARGETS:

  $ oc get hpa
  NAME         REFERENCE                      TARGETS         MINPODS   MAXPODS   REPLICAS   AGE
  <hpa_name>   Deployment/<deployment_name>   <unknown>/40%   1         10        1          3m23s
  

• The Horizontal Pod Autoscaler (HPA) metrics shows <unknown> for the current usage and error messages in the conditions:

  $ oc describe hpa <hpa_name>
  [...]
  Metrics:                                               ( current / target )
    resource cpu on pods  (as a percentage of request):  <unknown> / 50%
  [...]
  Conditions:
    Type           Status  Reason                   Message
    ----           ------  ------                   -------
    AbleToScale    True    SucceededGetScale        the HPA controller was able to get the target's current scale
    ScalingActive  False   FailedGetResourceMetric  the HPA was unable to compute the replica count: unable to get metrics for resource cpu: no metrics returned from resource metrics API
  [...]
  

  $ oc get hpa <hpa_name> -o yaml
  [...]
  conditions:
  - type: AbleToScale
    status: 'True'
    lastTransitionTime: '2020-06-30T15:05:26Z'
    reason: SucceededGetScale
    message: the HPA controller was able to get the target's current scale
  - type: ScalingActive
    status: 'False'
    lastTransitionTime: '2020-06-30T15:05:26Z'
    reason: FailedGetResourceMetric
    message: 'the HPA was unable to compute the replica count: missing request for cpu'
  [...]
  

• The HorizontalPodAutoscaler was defined with apiVersion: autoscaling/v1 and targetCPUUtilizationPercentage