Update the metrics API to support multiple metric values

[rakshitgondwal]

Description

In order to have multiple metric values, we need to update our metrics API/ KeptnMetric as follows:

apiVersion: metrics.keptn.sh/v1alpha3
kind: KeptnMetric
metadata:
  name: podtato-head
spec:
  provider:
    name: "my-provider"
  query: "sum(kube_pod_container_resource_limits{resource='cpu'})"
  fetchIntervalSeconds: 5
  range:
    interval: "5m"
    step: "1m"
    aggregation: "p90"
    storedResults: 5
status:
  value: "1.0"
  rawValue: "{}"
  lastUpdated: ....
  errMsg: nil
  intervalResults:
  - value: 1
    range:
      from: ""
      to: ""
      aggregation: max
    lastUpdated: ...
    errMsg: nil
  - value: 1
    range:
      from: ""
      to: ""
      aggregation: max

DoD:

• Add spec.range.storedResult field.
• Modify the status, to have a slice of IntervalResult type structs.