func (ls Labels) WithoutEmpty() Labels {
	for _, v := range ls {
		if v.Value == "" {
			els := make(Labels, 0, len(ls)-1)
			for _, v := range ls {
				if v.Value != "" {
					els = append(els, v)
				}
			}
			return els
		}
	}
	return ls
}
WithoutEmpty returns the labelset without empty labels.
May return the same labelset.
package labels
import (
	"bufio"
	"bytes"
	"os"
	"sort"
	"strconv"
	"strings"
	"github.com/cespare/xxhash"
	"github.com/pkg/errors"
)
const sep = '\xff'
type Label struct {
	Name, Value string
}
type Labels []Label
func (ls Labels) Len() int           { return len(ls) }
func (ls Labels) Swap(i, j int)      { ls[i], ls[j] = ls[j], ls[i] }
func (ls Labels) Less(i, j int) bool { return ls[i].Name < ls[j].Name }
func (ls Labels) String() string {
	var b bytes.Buffer
	b.WriteByte('{')
	for i, l := range ls {
		if i > 0 {
			b.WriteByte(',')
		}
		b.WriteString(l.Name)
		b.WriteByte('=')
		b.WriteString(strconv.Quote(l.Value))
	}
	b.WriteByte('}')
	return b.String()
}
func (ls Labels) Hash() uint64 {
	b := make([]byte, 0, 1024)
	for _, v := range ls {
		b = append(b, v.Name...)
		b = append(b, sep)
		b = append(b, v.Value...)
		b = append(b, sep)
	}
	return xxhash.Sum64(b)
}
func (ls Labels) Get(name string) string {
	for _, l := range ls {
		if l.Name == name {
			return l.Value
		}
	}
	return ""
}
func (ls Labels) Equals(o Labels) bool {
	if len(ls) != len(o) {
		return false
	}
	for i, l := range ls {
		if o[i] != l {
			return false
		}
	}
	return true
}
func (ls Labels) Map() map[string]string {
	m := make(map[string]string, len(ls))
	for _, l := range ls {
		m[l.Name] = l.Value
	}
	return m
}