* Renamed `MetricNamer` to `SeriesConverter` and renamed `MetricNameConverter` to `MetricNamer`.
* Simplified the `metricConverter` code.
* Greatly simplified the `externalSeriesRegistry` and removed the `externalInfoMap` code.
* Fixed doc comment format.
* Still several `TODO`s to address.
* 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.
* Breaking out some types to make the functionality more composable and easier to digest. (e.g. `basicMetricLister` interacts with Prometheus and `periodicMetricLister` periodically invokes `basicMetricLister`.
* Pulling out some of the type embedding between `basicSeriesRegistry` and `MetricLister` to make it easier to digest.
* Deleting the `/metric-converter` code because I'm pretty certain it's not going to be necessary as things transition to using the namer-based configuration.
* Some light-ish refactoring in `metricNamer` to get some re-use out of query generation in preparation for using it with external metrics.
This commit introduces advanced configuration. The rate-interval and
label-prefix flags are removed, and replaced by a configuration file
that allows you to specify series queries and the rules for transforming
those into metrics queries and API resources.
