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/vector_converter.go

@@ -0,0 +1,58 @@
+package provider
+
+import (
+	"errors"
+
+	prom "github.com/directxman12/k8s-prometheus-adapter/pkg/client"
+	"github.com/prometheus/common/model"
+	"k8s.io/metrics/pkg/apis/external_metrics"
+)
+
+type vectorConverter struct {
+	SampleConverter SampleConverter
+}
+
+//NewVectorConverter creates a VectorConverter capable of converting
+//vector Prometheus query results into external metric types.
+func NewVectorConverter(sampleConverter *SampleConverter) MetricConverter {
+	return &vectorConverter{
+		SampleConverter: *sampleConverter,
+	}
+}
+
+func (c *vectorConverter) Convert(queryResult prom.QueryResult) (*external_metrics.ExternalMetricValueList, error) {
+	if queryResult.Type != model.ValVector {
+		return nil, errors.New("vectorConverter can only convert scalar query results")
+	}
+
+	toConvert := *queryResult.Vector
+
+	if toConvert == nil {
+		return nil, errors.New("the provided input did not contain vector query results")
+	}
+
+	return c.convert(toConvert)
+}
+
+func (c *vectorConverter) convert(result model.Vector) (*external_metrics.ExternalMetricValueList, error) {
+	items := []external_metrics.ExternalMetricValue{}
+	metricValueList := external_metrics.ExternalMetricValueList{
+		Items: items,
+	}
+
+	numSamples := result.Len()
+	if numSamples == 0 {
+		return &metricValueList, nil
+	}
+
+	for _, val := range result {
+		//TODO: Care about potential errors here.
+		singleMetric, _ := c.SampleConverter.Convert(val)
+		items = append(items, *singleMetric)
+	}
+
+	metricValueList = external_metrics.ExternalMetricValueList{
+		Items: items,
+	}
+	return &metricValueList, nil
+}