plugin/metadata: add metadata plugin (#1894)

* plugin/metadata: add metadata plugin

* plugin/metadata: Add MD struct, refactor code, fix doc

* plugin/metadata: simplify metadata key

* plugin/metadata: improve setup_test

* Support of metadata by rewrite plugin. Move calculated variables to metadata.

* Move variables from metadata to pkg, add UTs, READMEs change, metadata small fixes

* Add client port validation to variables_test

* plugin/metadata: improve README

* plugin/metadata: rename methods

* plugin/metadata: Update Metadataer interface, update doc, cosmetic code changes

* plugin/metadata: move colllisions check to OnStartup(). Fix default variables metadataer.

* plugin/metadata: Fix comment for method setValue

* plugin/metadata: change variables order to fix linter warning

* plugin/metadata: rename Metadataer to Provider

plugin/metadata/metadata.go

@@ -0,0 +1,55 @@
+package metadata
+
+import (
+	"context"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/pkg/variables"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+// Metadata implements collecting metadata information from all plugins that
+// implement the Provider interface.
+type Metadata struct {
+	Zones     []string
+	Providers []Provider
+	Next      plugin.Handler
+}
+
+// Name implements the Handler interface.
+func (m *Metadata) Name() string { return "metadata" }
+
+// ServeDNS implements the plugin.Handler interface.
+func (m *Metadata) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+
+	md, ctx := newMD(ctx)
+
+	state := request.Request{W: w, Req: r}
+	if plugin.Zones(m.Zones).Matches(state.Name()) != "" {
+		// Go through all Providers and collect metadata
+		for _, provider := range m.Providers {
+			for _, varName := range provider.MetadataVarNames() {
+				if val, ok := provider.Metadata(ctx, w, r, varName); ok {
+					md.setValue(varName, val)
+				}
+			}
+		}
+	}
+
+	rcode, err := plugin.NextOrFailure(m.Name(), m.Next, ctx, w, r)
+
+	return rcode, err
+}
+
+// MetadataVarNames implements the plugin.Provider interface.
+func (m *Metadata) MetadataVarNames() []string { return variables.All }
+
+// Metadata implements the plugin.Provider interface.
+func (m *Metadata) Metadata(ctx context.Context, w dns.ResponseWriter, r *dns.Msg, varName string) (interface{}, bool) {
+	if val, err := variables.GetValue(varName, w, r); err == nil {
+		return val, true
+	}
+	return nil, false
+}