Breaking down some more components, adding tests.

* Some bug fixes found during testing/test repair.
* Trying to tease apart the various responsibilities of `metricNamer` into smaller chunks and adding tests for each individual chunk.
* Updating the `provider_test` and `series_registry_test` to fix failures.

pkg/custom-provider/metric_name_converter.go

@@ -0,0 +1,60 @@
+package provider
+
+import (
+	"fmt"
+	"regexp"
+
+	prom "github.com/directxman12/k8s-prometheus-adapter/pkg/client"
+	"github.com/directxman12/k8s-prometheus-adapter/pkg/config"
+)
+
+type MetricNameConverter interface {
+	GetMetricNameForSeries(series prom.Series) (string, error)
+}
+
+type metricNameConverter struct {
+	nameMatches *regexp.Regexp
+	nameAs      string
+}
+
+func NewMetricNameConverter(mapping config.NameMapping) (MetricNameConverter, error) {
+	var nameMatches *regexp.Regexp
+	var err error
+	if mapping.Matches != "" {
+		nameMatches, err = regexp.Compile(mapping.Matches)
+		if err != nil {
+			return nil, fmt.Errorf("unable to compile series name match expression %q: %v", mapping.Matches, err)
+		}
+	} else {
+		// this will always succeed
+		nameMatches = regexp.MustCompile(".*")
+	}
+	nameAs := mapping.As
+	if nameAs == "" {
+		// check if we have an obvious default
+		subexpNames := nameMatches.SubexpNames()
+		if len(subexpNames) == 1 {
+			// no capture groups, use the whole thing
+			nameAs = "$0"
+		} else if len(subexpNames) == 2 {
+			// one capture group, use that
+			nameAs = "$1"
+		} else {
+			return nil, fmt.Errorf("must specify an 'as' value for name matcher %q", mapping.Matches)
+		}
+	}
+
+	return &metricNameConverter{
+		nameMatches: nameMatches,
+		nameAs:      nameAs,
+	}, nil
+}
+
+func (c *metricNameConverter) GetMetricNameForSeries(series prom.Series) (string, error) {
+	matches := c.nameMatches.FindStringSubmatchIndex(series.Name)
+	if matches == nil {
+		return "", fmt.Errorf("series name %q did not match expected pattern %q", series.Name, c.nameMatches.String())
+	}
+	outNameBytes := c.nameMatches.ExpandString(nil, c.nameAs, series.Name, matches)
+	return string(outNameBytes), nil
+}