Stop logging client disconnect as a pilot error

Right now when Envoy shuts down it sometimes sends an Unavailable error
code instead of Cancelled - see
etcd-io/etcd#10289 for details, but its pretty
complex. The end result is this shows up as "internal error" in the
dashboards, when there is nothing going wrong. Instead, we should filter
these out.

Checking the string directly feels a bit like a hack, but etcd does it
so seems reasonable:
https://github.com/etcd-io/etcd/blob/master/etcdserver/api/v3rpc/util.go#L113

pilot/pkg/proxy/envoy/v2/ads.go

@@ -142,12 +142,29 @@ func newXdsConnection(peerAddr string, stream DiscoveryStream) *XdsConnection {
 	}
 }
 
+// isExpectedGRPCError checks a gRPC error code and determines whether it is an expected error when
+// things are operating normally. This is basically capturing when the client disconnects.
+func isExpectedGRPCError(err error) bool {
+	if err == io.EOF {
+		return true
+	}
+
+	s := status.Convert(err)
+	if s.Code() == codes.Canceled {
+		return true
+	}
+	if s.Message() == "client disconnected" {
+		return true
+	}
+	return false
+}
+
 func receiveThread(con *XdsConnection, reqChannel chan *xdsapi.DiscoveryRequest, errP *error) {
 	defer close(reqChannel) // indicates close of the remote side.
 	for {
 		req, err := con.stream.Recv()
 		if err != nil {
-			if status.Code(err) == codes.Canceled || err == io.EOF {
+			if isExpectedGRPCError(err) {
 				con.mu.RLock()
 				adsLog.Infof("ADS: %q %s terminated %v", con.PeerAddr, con.ConID, err)
 				con.mu.RUnlock()