Cleaning up.

* Wiped out a ton of warnings about comments.
* Created consts for `MetricType` values.
* `externalInfoMap` can now track multiple series with the same name/namespace and different labels.
* `namespace` parameter of external metrics queries is now respected (albeit very rudimentarily)
* Metric converter values for external metrics are now converted the same way as for custom metrics (probably still some opportunity for consolidation).
* Lots of TODOs actually done.
* Deleted a lot of commented out code.

pkg/custom-provider/query_builder.go

@@ -9,6 +9,7 @@ import (
 	prom "github.com/directxman12/k8s-prometheus-adapter/pkg/client"
 )
 
+//QueryBuilder provides functions for generating Prometheus queries.
 type QueryBuilder interface {
 	BuildSelector(seriesName string, groupBy string, groupBySlice []string, queryParts []queryPart) (prom.Selector, error)
 }
@@ -17,6 +18,7 @@ type queryBuilder struct {
 	metricsQueryTemplate *template.Template
 }
 
+//NewQueryBuilder creates a QueryBuilder.
 func NewQueryBuilder(metricsQuery string) (QueryBuilder, error) {
 	metricsQueryTemplate, err := template.New("metrics-query").Delims("<<", ">>").Parse(metricsQuery)
 	if err != nil {