plugin/metadata: metadata is just label=value (#1914)

This revert 17d807f0 and re-adds the metadata plugin as a plugin that
just sets a label to a value function.

Add package documentation on how to use the metadata package. Make it
clear that any caching is up to the Func implemented.

There are now - no in tree users. We could add the request metadata by
default under names that copy request.Request, i.e

request/ip - remote IP
request/port - remote port

Variables.go has been deleted.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/metadata/metadata.go

@@ -4,7 +4,6 @@ import (
 	"context"
 
 	"github.com/coredns/coredns/plugin"
-	"github.com/coredns/coredns/plugin/pkg/variables"
 	"github.com/coredns/coredns/request"
 
 	"github.com/miekg/dns"
@@ -24,18 +23,13 @@ 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) {
 
-	ctx = context.WithValue(ctx, metadataKey{}, M{})
-	md, _ := FromContext(ctx)
+	ctx = context.WithValue(ctx, key{}, md{})
 
 	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, state, varName); ok {
-					md.SetValue(varName, val)
-				}
-			}
+		for _, p := range m.Providers {
+			ctx = p.Metadata(ctx, state)
 		}
 	}
 
@@ -43,14 +37,3 @@ func (m *Metadata) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Ms
 
 	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, state request.Request, varName string) (interface{}, bool) {
-	if val, err := variables.GetValue(state, varName); err == nil {
-		return val, true
-	}
-	return nil, false
-}