plugin/rewrite - extend edns0 local variable support with metadata (#1928)

* - add support of metadata values for edns0 local variables

* - comments from review.

* - simplify label check. Add UT

* - enhance check for Labels, add UT
- remove IsMetadataSet

* - edns0 variable - if variable is not found just ignore the rewrite.

plugin/metadata/provider.go

@@ -32,6 +32,7 @@ package metadata
 
 import (
 	"context"
+	"strings"
 
 	"github.com/coredns/coredns/request"
 )
@@ -48,6 +49,21 @@ type Provider interface {
 // Func is the type of function in the metadata, when called they return the value of the label.
 type Func func() string
 
+// IsLabel check that the provided name looks like a valid label name
+func IsLabel(label string) bool {
+	p := strings.Index(label, "/")
+	if p <= 0 || p >= len(label)-1 {
+		// cannot accept namespace empty nor label empty
+		return false
+	}
+	if strings.LastIndex(label, "/") != p {
+		// several slash in the Label
+		return false
+	}
+	return true
+
+}
+
 // Labels returns all metadata keys stored in the context. These label names should be named
 // as: plugin/NAME, where NAME is something descriptive.
 func Labels(ctx context.Context) []string {