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/metric_converter.go

@@ -0,0 +1,52 @@
+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"
+)
+
+//MetricConverter provides a unified interface for converting the results of
+//Prometheus queries into external metric types.
+type MetricConverter interface {
+	Convert(queryResult prom.QueryResult) (*external_metrics.ExternalMetricValueList, error)
+}
+
+type metricConverter struct {
+	scalarConverter MetricConverter
+	vectorConverter MetricConverter
+	matrixConverter MetricConverter
+}
+
+func DefaultMetricConverter() MetricConverter {
+	sampleConverter := NewSampleConverter()
+	return NewMetricConverter(NewScalarConverter(), NewVectorConverter(&sampleConverter), NewMatrixConverter())
+}
+
+//NewMetricConverter creates a MetricCoverter, capable of converting any of the three metric types
+//returned by the Prometheus client into external metrics types.
+func NewMetricConverter(scalar MetricConverter, vector MetricConverter, matrix MetricConverter) MetricConverter {
+	return &metricConverter{
+		scalarConverter: scalar,
+		vectorConverter: vector,
+		matrixConverter: matrix,
+	}
+}
+
+func (c *metricConverter) Convert(queryResult prom.QueryResult) (*external_metrics.ExternalMetricValueList, error) {
+	if queryResult.Type == model.ValScalar {
+		return c.scalarConverter.Convert(queryResult)
+	}
+
+	if queryResult.Type == model.ValVector {
+		return c.vectorConverter.Convert(queryResult)
+	}
+
+	if queryResult.Type == model.ValMatrix {
+		return c.matrixConverter.Convert(queryResult)
+	}
+
+	return nil, errors.New("encountered an unexpected query result type")
+}