func NewOutgoingContext(ctx context.Context, md MD) context.Context {
	return context.WithValue(ctx, mdOutgoingKey{}, md)
}
NewOutgoingContext creates a new context with outgoing md attached.
package metadata  
import (
	"fmt"
	"strings"
	"golang.org/x/net/context"
)
func DecodeKeyValue(k, v string) (string, string, error) {
	return k, v, nil
}
type MD map[string][]string
func New(m map[string]string) MD {
	md := MD{}
	for k, val := range m {
		key := strings.ToLower(k)
		md[key] = append(md[key], val)
	}
	return md
}
func Pairs(kv ...string) MD {
	if len(kv)%2 == 1 {
		panic(fmt.Sprintf("metadata: Pairs got the odd number of input pairs for metadata: %d", len(kv)))
	}
	md := MD{}
	var key string
	for i, s := range kv {
		if i%2 == 0 {
			key = strings.ToLower(s)
			continue
		}
		md[key] = append(md[key], s)
	}
	return md
}
func (md MD) Len() int {
	return len(md)
}
func (md MD) Copy() MD {
	return Join(md)
}
func Join(mds ...MD) MD {
	out := MD{}
	for _, md := range mds {
		for k, v := range md {
			out[k] = append(out[k], v...)
		}
	}
	return out
}
type mdIncomingKey struct{}
type mdOutgoingKey struct{}
func NewIncomingContext(ctx context.Context, md MD) context.Context {
	return context.WithValue(ctx, mdIncomingKey{}, md)
}