Add _grpc_config replies

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

plugin/traffic/lb.go

@@ -0,0 +1,14 @@
+package traffic
+
+import "github.com/miekg/dns"
+
+// See https://github.com/grpc/grpc/blob/master/doc/service_config.md for the fields in this proto.
+// We encode it as json and return it in a TXT field.
+var lbTXT = `grpc_config=[{"serviceConfig":{"loadBalancingConfig":[{"xds_experimental":{"lrs_load_reporting_server_name":""}}]}}]`
+
+func txt(z string) []dns.RR {
+	return []dns.RR{&dns.TXT{
+		Hdr: dns.RR_Header{Name: z, Rrtype: dns.TypeTXT, Class: dns.ClassINET, Ttl: 5},
+		Txt: []string{lbTXT},
+	}}
+}