func (r *response) Header() map[string]string {
	md, err := r.stream.Header()
	if err != nil {
		return map[string]string{}
	}
	hdr := make(map[string]string, len(md))
	for k, v := range md {
		hdr[k] = strings.Join(v, ",")
	}
	return hdr
}
read the header
package grpc
import (
	"encoding/json"
	"strings"
	"github.com/micro/micro/v3/service/errors"
	"github.com/micro/micro/v3/util/codec"
	"github.com/micro/micro/v3/util/codec/bytes"
	"google.golang.org/grpc"
	"google.golang.org/grpc/encoding"
	"google.golang.org/grpc/status"
)
type response struct {
	conn   *poolConn
	stream grpc.ClientStream
	codec  encoding.Codec
	gcodec codec.Codec
}
func (r *response) Codec() codec.Reader {
	return r.gcodec
}