* 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.
Notable next steps include:
* Intelligently select the aggregation method (rate/avg_over_time/etc).
* Intelligenty select the aggregation window.
* Figure out how to handle "namespace", if at all.
* Fix the crazy type conversion in `sample_converter.go`.
Working through some of the result conversions.
I want to use this code in a harness project to exercise some of it, but there are some versioning issues around my modified code vs. the current version of the real project. The easiest way to work around them at the moment is to push this.
