func (ls Set) AsSelector() Selector {
	return SelectorFromSet(ls)
}
AsSelector converts labels into a selectors.
package labels
import (
	"fmt"
	"sort"
	"strings"
)
type Labels interface {
	Has(label string) (exists bool)
	Get(label string) (value string)
}
type Set map[string]string
func (ls Set) String() string {
	selector := make([]string, 0, len(ls))
	for key, value := range ls {
		selector = append(selector, key+"="+value)
	}
	sort.StringSlice(selector).Sort()
	return strings.Join(selector, ",")
}
func (ls Set) Has(label string) bool {
	_, exists := ls[label]
	return exists
}
func (ls Set) Get(label string) string {
	return ls[label]
}