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/setup.go

@@ -1,8 +1,6 @@
 package metadata
 
 import (
-	"fmt"
-
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/plugin"
 
@@ -28,16 +26,8 @@ func setup(c *caddy.Controller) error {
 
 	c.OnStartup(func() error {
 		plugins := dnsserver.GetConfig(c).Handlers()
-		// Collect all plugins which implement Provider interface
-		metadataVariables := map[string]bool{}
 		for _, p := range plugins {
 			if met, ok := p.(Provider); ok {
-				for _, varName := range met.MetadataVarNames() {
-					if _, ok := metadataVariables[varName]; ok {
-						return fmt.Errorf("Metadata variable '%v' has duplicates", varName)
-					}
-					metadataVariables[varName] = true
-				}
 				m.Providers = append(m.Providers, met)
 			}
 		}