* 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.
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.
