plugin/view: Advanced routing interface and new 'view' plugin (#5538)

* introduce new interface "dnsserver.Viewer", that allows a plugin implementing it to decide if a query should be routed into its server block.
* add new plugin "view", that uses the new interface to enable a user to define expression based conditions that must be met for a query to be routed to its server block.

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/metadata/metadata_test.go

@@ -47,7 +47,10 @@ func TestMetadataServeDNS(t *testing.T) {
 	}
 
 	ctx := context.TODO()
-	m.ServeDNS(ctx, &test.ResponseWriter{}, new(dns.Msg))
+	w := &test.ResponseWriter{}
+	r := new(dns.Msg)
+	ctx = m.Collect(ctx, request.Request{W: w, Req: r})
+	m.ServeDNS(ctx, w, r)
 	nctx := next.ctx
 
 	for _, expected := range expectedMetadata {