Fixing some refactoring bugs, first half-decent external metrics attempt.

Fixed:
* `basicMetricLister` wasn't applying the appropriate start time because I had forgotten to set the `lookback`.

There are still a number of issues:
* The `externalPrometheusProvider` is not hooked up to the web application yet, so it doesn't serve requests.
* The namespace and label approach used in `external_info_map.go` is horrifically incorrect. It doesn't appropriately store multiple series with the same name but different labels.
* The configuration is still not updated to appropriately handle external metrics, it's sort of half-piggy-backing on the pre-existing work.

pkg/custom-provider/metric-converter/scalar_converter.go

@@ -0,0 +1,55 @@
+package provider
+
+import (
+	"errors"
+
+	prom "github.com/directxman12/k8s-prometheus-adapter/pkg/client"
+	"github.com/prometheus/common/model"
+	"k8s.io/apimachinery/pkg/api/resource"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/metrics/pkg/apis/external_metrics"
+)
+
+type scalarConverter struct {
+}
+
+//NewScalarConverter creates a ScalarConverter capable of converting
+//scalar Prometheus query results into external metric types.
+func NewScalarConverter() MetricConverter {
+	return &scalarConverter{}
+}
+
+func (c *scalarConverter) Convert(queryResult prom.QueryResult) (*external_metrics.ExternalMetricValueList, error) {
+	if queryResult.Type != model.ValScalar {
+		return nil, errors.New("scalarConverter can only convert scalar query results")
+	}
+
+	toConvert := queryResult.Scalar
+
+	if toConvert == nil {
+		return nil, errors.New("the provided input did not contain scalar query results")
+	}
+
+	return c.convert(toConvert)
+}
+
+func (c *scalarConverter) convert(input *model.Scalar) (*external_metrics.ExternalMetricValueList, error) {
+	result := external_metrics.ExternalMetricValueList{
+		//Using prometheusProvider.metricsFor(...) as an example,
+		//it seems that I don't need to provide values for
+		//TypeMeta and ListMeta.
+		//TODO: Get some confirmation on this.
+		Items: []external_metrics.ExternalMetricValue{
+			{
+				Timestamp: metav1.Time{
+					input.Timestamp.Time(),
+				},
+				//TODO: I'm not so sure about this type/conversions.
+				//Is there a meaningful loss of precision here?
+				//Does K8S only deal win integer metrics?
+				Value: *resource.NewMilliQuantity(int64(input.Value*1000.0), resource.DecimalSI),
+			},
+		},
+	}
+	return &result, nil
+}