Publish metadata from kubernetes plugin (#2829)

* Publish metadata from kubernetes plugin

* stickler fix

* Add a couple tests

* Add metadata section to README

* Update plugin/kubernetes/README.md

Co-Authored-By: Chris O'Haver <cohaver@infoblox.com>

* Address nit

plugin/kubernetes/metadata.go

@@ -0,0 +1,59 @@
+package kubernetes
+
+import (
+	"context"
+
+	"github.com/coredns/coredns/plugin/metadata"
+	"github.com/coredns/coredns/request"
+)
+
+// Metadata implements the metadata.Provider interface.
+func (k *Kubernetes) Metadata(ctx context.Context, state request.Request) context.Context {
+	// possible optimization: cache r so it doesn't need to be calculated again in ServeDNS
+	r, err := parseRequest(state)
+	if err != nil {
+		metadata.SetValueFunc(ctx, "kubernetes/parse-error", func() string {
+			return err.Error()
+		})
+		return ctx
+	}
+
+	metadata.SetValueFunc(ctx, "kubernetes/port-name", func() string {
+		return r.port
+	})
+
+	metadata.SetValueFunc(ctx, "kubernetes/protocol", func() string {
+		return r.protocol
+	})
+
+	metadata.SetValueFunc(ctx, "kubernetes/endpoint", func() string {
+		return r.endpoint
+	})
+
+	metadata.SetValueFunc(ctx, "kubernetes/service", func() string {
+		return r.service
+	})
+
+	metadata.SetValueFunc(ctx, "kubernetes/namespace", func() string {
+		return r.namespace
+	})
+
+	metadata.SetValueFunc(ctx, "kubernetes/kind", func() string {
+		return r.podOrSvc
+	})
+
+	pod := k.podWithIP(state.IP())
+	if pod == nil {
+		return ctx
+	}
+
+	metadata.SetValueFunc(ctx, "kubernetes/client-namespace", func() string {
+		return pod.Namespace
+	})
+
+	metadata.SetValueFunc(ctx, "kubernetes/client-pod-name", func() string {
+		return pod.Name
+	})
+
+	return ctx
+}