func (md MD) Set(k string, vals ...string) {
	if len(vals) == 0 {
		return
	}
	k = strings.ToLower(k)
	md[k] = vals
}
Set sets the value of a given key with a slice of values.
package metadata  
import (
	"context"
	"fmt"
	"strings"
)
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 (md MD) Get(k string) []string {
	k = strings.ToLower(k)
	return md[k]
}