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/regex_matcher.go

@@ -0,0 +1,47 @@
+package provider
+
+import (
+	"fmt"
+	"regexp"
+
+	"github.com/directxman12/k8s-prometheus-adapter/pkg/config"
+)
+
+// reMatcher either positively or negatively matches a regex
+type reMatcher struct {
+	regex    *regexp.Regexp
+	positive bool
+}
+
+func newReMatcher(cfg config.RegexFilter) (*reMatcher, error) {
+	if cfg.Is != "" && cfg.IsNot != "" {
+		return nil, fmt.Errorf("cannot have both an `is` (%q) and `isNot` (%q) expression in a single filter", cfg.Is, cfg.IsNot)
+	}
+	if cfg.Is == "" && cfg.IsNot == "" {
+		return nil, fmt.Errorf("must have either an `is` or `isNot` expression in a filter")
+	}
+
+	var positive bool
+	var regexRaw string
+	if cfg.Is != "" {
+		positive = true
+		regexRaw = cfg.Is
+	} else {
+		positive = false
+		regexRaw = cfg.IsNot
+	}
+
+	regex, err := regexp.Compile(regexRaw)
+	if err != nil {
+		return nil, fmt.Errorf("unable to compile series filter %q: %v", regexRaw, err)
+	}
+
+	return &reMatcher{
+		regex:    regex,
+		positive: positive,
+	}, nil
+}
+
+func (m *reMatcher) Matches(val string) bool {
+	return m.regex.MatchString(val) == m.positive
+}