log/forward plugins: Extend dns query logging (#2240)

Automatically submitted.

plugin/forward/README.md

@@ -91,6 +91,10 @@ On each endpoint, the timeouts of the communication are set by default and autom
 * dialTimeout by default is 30 sec, and can decrease automatically down to 100ms
 * readTimeout by default is 2 sec, and can decrease automatically down to 200ms
 
+## Metadata
+
+* forward/resolving_proxy : provide the IP address and port of the upstream resolver used to resolve the current DNS query.
+
 ## Metrics
 
 If monitoring is enabled (via the *prometheus* directive) then the following metric are exported:

plugin/forward/forward.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/debug"
+	"github.com/coredns/coredns/plugin/metadata"
 	clog "github.com/coredns/coredns/plugin/pkg/log"
 	"github.com/coredns/coredns/request"
 
@@ -59,6 +60,15 @@ func (f *Forward) Len() int { return len(f.proxies) }
 // Name implements plugin.Handler.
 func (f *Forward) Name() string { return "forward" }
 
+//declareMetadata adds to the context a metadata parameter which will return the passed value.
+func (f *Forward) declareMetadata(ctx context.Context, name string, value string) bool {
+	label := f.Name()+"/"+name
+	if metadata.IsLabel(label) {
+		return metadata.SetValueFunc(ctx, label, func() string { return value })
+	}
+	return false
+}
+
 // ServeDNS implements plugin.Handler.
 func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
 
@@ -148,10 +158,12 @@ func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 
 			formerr := state.ErrorMessage(dns.RcodeFormatError)
 			w.WriteMsg(formerr)
+			f.declareMetadata(ctx, "resolving_proxy", proxy.addr)
 			return 0, nil
 		}
 
 		w.WriteMsg(ret)
+		f.declareMetadata(ctx, "resolving_proxy", proxy.addr)
 		return 0, nil
 	}