func Delete(ctx context.Context, k string) context.Context {
	return Set(ctx, k, "")
}
Delete key from metadata
package metadata
import (
	"context"
	"strings"
)
type metadataKey struct{}
type Metadata map[string]string
func (md Metadata) Get(key string) (string, bool) {
	val, ok := md[key]
	if ok {
		return val, ok
	}
	val, ok = md[strings.Title(key)]
	return val, ok
}
func (md Metadata) Set(key, val string) {
	md[key] = val
}
func (md Metadata) Delete(key string) {
	delete(md, key)
	delete(md, strings.Title(key))
}
func Copy(md Metadata) Metadata {
	cmd := make(Metadata, len(md))
	for k, v := range md {
		cmd[k] = v
	}
	return cmd
}