func (m Metadata) Range(f func(k, v string) bool) {
	for k, v := range m {
		ret := f(k, v)
		if !ret {
			break
		}
	}
}
Range iterate over element in metadata.
package metadata
import (
	"context"
	"fmt"
	"strings"
)
type Metadata map[string]string
func NewMetadata(mds ...map[string]string) Metadata {
	md := Metadata{}
	for _, m := range mds {
		for k, v := range m {
			md.Set(k, v)
		}
	}
	return md
}
func (m Metadata) GetValue(key string) string {
	k := strings.ToLower(key)
	return m[k]
}
func (m Metadata) Set(key string, value string) {
	if key == "" || value == "" {
		return
	}
	k := strings.ToLower(key)
	m[k] = value
}