Extract label-resource logic to separate package

This extracts the label-to-resource conversion to a separte "naming" package for
use across different providers.

Early versions of the commit were done by antoinne85 in #79.

pkg/naming/lbl_res.go

@@ -0,0 +1,83 @@
+package naming
+
+import (
+	"bytes"
+	"fmt"
+	"regexp"
+	"text/template"
+
+	apimeta "k8s.io/apimachinery/pkg/api/meta"
+	"k8s.io/apimachinery/pkg/runtime/schema"
+
+	pmodel "github.com/prometheus/common/model"
+)
+
+// labelGroupResExtractor extracts schema.GroupResources from series labels.
+type labelGroupResExtractor struct {
+	regex *regexp.Regexp
+
+	resourceInd int
+	groupInd    *int
+	mapper      apimeta.RESTMapper
+}
+
+// newLabelGroupResExtractor creates a new labelGroupResExtractor for labels whose form
+// matches the given template.  It does so by creating a regular expression from the template,
+// so anything in the template which limits resource or group name length will cause issues.
+func newLabelGroupResExtractor(labelTemplate *template.Template) (*labelGroupResExtractor, error) {
+	labelRegexBuff := new(bytes.Buffer)
+	if err := labelTemplate.Execute(labelRegexBuff, schema.GroupResource{"(?P<group>.+?)", "(?P<resource>.+?)"}); err != nil {
+		return nil, fmt.Errorf("unable to convert label template to matcher: %v", err)
+	}
+	if labelRegexBuff.Len() == 0 {
+		return nil, fmt.Errorf("unable to convert label template to matcher: empty template")
+	}
+	labelRegexRaw := "^" + labelRegexBuff.String() + "$"
+	labelRegex, err := regexp.Compile(labelRegexRaw)
+	if err != nil {
+		return nil, fmt.Errorf("unable to convert label template to matcher: %v", err)
+	}
+
+	var groupInd *int
+	var resInd *int
+
+	for i, name := range labelRegex.SubexpNames() {
+		switch name {
+		case "group":
+			ind := i // copy to avoid iteration variable reference
+			groupInd = &ind
+		case "resource":
+			ind := i // copy to avoid iteration variable reference
+			resInd = &ind
+		}
+	}
+
+	if resInd == nil {
+		return nil, fmt.Errorf("must include at least `{{.Resource}}` in the label template")
+	}
+
+	return &labelGroupResExtractor{
+		regex:       labelRegex,
+		resourceInd: *resInd,
+		groupInd:    groupInd,
+	}, nil
+}
+
+// GroupResourceForLabel extracts a schema.GroupResource from the given label, if possible.
+// The second return value indicates whether or not a potential group-resource was found in this label.
+func (e *labelGroupResExtractor) GroupResourceForLabel(lbl pmodel.LabelName) (schema.GroupResource, bool) {
+	matchGroups := e.regex.FindStringSubmatch(string(lbl))
+	if matchGroups != nil {
+		group := ""
+		if e.groupInd != nil {
+			group = matchGroups[*e.groupInd]
+		}
+
+		return schema.GroupResource{
+			Group:    group,
+			Resource: matchGroups[e.resourceInd],
+		}, true
+	}
+
+	return schema.GroupResource{}, false
+}