Tons of movement. See notes.

* 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.

pkg/custom-provider/errors.go

@@ -0,0 +1,27 @@
+package provider
+
+import "errors"
+
+// NewOperatorNotSupportedByPrometheusError creates an error that represents the fact that we were requested to service a query that
+// Prometheus would be unable to support.
+func NewOperatorNotSupportedByPrometheusError() error {
+	return errors.New("operator not supported by prometheus")
+}
+
+// NewOperatorRequiresValuesError creates an error that represents the fact that we were requested to service a query
+// that was malformed in its operator/value combination.
+func NewOperatorRequiresValuesError() error {
+	return errors.New("operator requires values")
+}
+
+// NewOperatorDoesNotSupportValuesError creates an error that represents the fact that we were requested to service a query
+// that was malformed in its operator/value combination.
+func NewOperatorDoesNotSupportValuesError() error {
+	return errors.New("operator does not support values")
+}
+
+// NewLabelNotSpecifiedError creates an error that represents the fact that we were requested to service a query
+// that was malformed in its label specification.
+func NewLabelNotSpecifiedError() error {
+	return errors.New("label not specified")
+}