pkg/external-provider: set metric name for scalar values

[s-urbaniak]

Currently simple external scalar values don't have a value attached:

{
  "kind": "ExternalMetricValueList",
  "apiVersion": "external.metrics.k8s.io/v1beta1",
  "metadata": {
    "selfLink": "/apis/external.metrics.k8s.io/v1beta1/namespaces/default/http_requests_per_second"
  },
  "items": [
    {
      "metricName": "",
..
      "value": "100133m"
    }
  ]
}

This fixes it:

{
  "kind": "ExternalMetricValueList",
  "apiVersion": "external.metrics.k8s.io/v1beta1",
  "metadata": {
    "selfLink": "/apis/external.metrics.k8s.io/v1beta1/namespaces/default/http_requests_per_second"
  },
  "items": [
    {
      "metricName": "http_requests_per_second",
...
      "value": "100133m"
    }
  ]
}

[s-urbaniak]

/cc @paulfantom

[metalmatze]

lgtm

[paulfantom]

rebased onto master and fixed unit test from #192