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/external_info_map.go

@@ -0,0 +1,108 @@
+package provider
+
+import (
+	prom "github.com/directxman12/k8s-prometheus-adapter/pkg/client"
+	"k8s.io/apimachinery/pkg/labels"
+)
+
+type ExportedMetric struct {
+	MetricName string
+	Labels     labels.Set
+	Namespace  string
+}
+
+type ExternalInfoMap interface {
+	TrackMetric(metricName string, generatedBy MetricNamer) ExternalMetricData
+	ExportMetrics() []ExportedMetric
+	FindMetric(metricName string) (data ExternalMetricData, found bool)
+}
+
+type ExternalMetricData interface {
+	MetricName() string
+	WithSeries(labels labels.Set)
+	WithNamespacedSeries(namespace string, labels labels.Set)
+	ExportMetrics() []ExportedMetric
+	GenerateQuery(selector labels.Selector) (prom.Selector, error)
+}
+
+type externalInfoMap struct {
+	metrics map[string]ExternalMetricData
+}
+
+type externalMetricData struct {
+	metricName     string
+	namespacedData map[string]labels.Set
+	generatedBy    MetricNamer
+}
+
+func NewExternalMetricData(metricName string, generatedBy MetricNamer) ExternalMetricData {
+	return &externalMetricData{
+		metricName:     metricName,
+		generatedBy:    generatedBy,
+		namespacedData: map[string]labels.Set{},
+	}
+}
+
+func NewExternalInfoMap() ExternalInfoMap {
+	return &externalInfoMap{
+		metrics: map[string]ExternalMetricData{},
+	}
+}
+
+func (i *externalInfoMap) ExportMetrics() []ExportedMetric {
+	results := make([]ExportedMetric, 0)
+	for _, info := range i.metrics {
+		exported := info.ExportMetrics()
+		results = append(results, exported...)
+	}
+
+	return results
+}
+
+func (i *externalInfoMap) FindMetric(metricName string) (data ExternalMetricData, found bool) {
+	data, found = i.metrics[metricName]
+	return data, found
+}
+
+func (i *externalInfoMap) TrackMetric(metricName string, generatedBy MetricNamer) ExternalMetricData {
+	data, found := i.metrics[metricName]
+	if !found {
+		data = NewExternalMetricData(metricName, generatedBy)
+		i.metrics[metricName] = data
+	}
+
+	return data
+}
+
+func (d *externalMetricData) MetricName() string {
+	return d.metricName
+}
+
+func (d *externalMetricData) GenerateQuery(selector labels.Selector) (prom.Selector, error) {
+	return d.generatedBy.QueryForExternalSeries(d.metricName, selector)
+}
+
+func (d *externalMetricData) ExportMetrics() []ExportedMetric {
+	results := make([]ExportedMetric, 0)
+	for namespace, labels := range d.namespacedData {
+		results = append(results, ExportedMetric{
+			Labels:     labels,
+			MetricName: d.metricName,
+			Namespace:  namespace,
+		})
+	}
+
+	return results
+}
+
+func (d *externalMetricData) WithSeries(labels labels.Set) {
+	d.WithNamespacedSeries("", labels)
+}
+
+func (d *externalMetricData) WithNamespacedSeries(namespace string, labels labels.Set) {
+	data, found := d.namespacedData[namespace]
+	if !found {
+		data = labels
+		d.namespacedData[namespace] = data
+	}
+}