func (u *UpstreamPorts) String() string {
	return strings.Join(strings.Fields(fmt.Sprint(*u)), ",")
}
String is a method that implements the flag.Value interface
package main
import (
	"context"
	"flag"
	"fmt"
	"strconv"
	"strings"
	"net"
	"sync"
	"sync/atomic"
	"time"
	core "github.com/envoyproxy/go-control-plane/envoy/config/core/v3"
	cluster "github.com/envoyproxy/go-control-plane/envoy/config/cluster/v3"
	endpoint "github.com/envoyproxy/go-control-plane/envoy/config/endpoint/v3"
	ep "github.com/envoyproxy/go-control-plane/envoy/config/endpoint/v3"
	listener "github.com/envoyproxy/go-control-plane/envoy/config/listener/v3"
	lv2 "github.com/envoyproxy/go-control-plane/envoy/config/listener/v3"
	route "github.com/envoyproxy/go-control-plane/envoy/config/route/v3"
	hcm "github.com/envoyproxy/go-control-plane/envoy/extensions/filters/network/http_connection_manager/v3"
	discovery "github.com/envoyproxy/go-control-plane/envoy/service/discovery/v3"
	"github.com/envoyproxy/go-control-plane/pkg/cache/types"
	cachev3 "github.com/envoyproxy/go-control-plane/pkg/cache/v3"
	xds "github.com/envoyproxy/go-control-plane/pkg/server/v3"
	"github.com/golang/protobuf/ptypes"
	log "github.com/sirupsen/logrus"
	"google.golang.org/grpc"
	"google.golang.org/protobuf/types/known/wrapperspb"
)
type UpstreamPorts []int