code: func (b *Builder) Set(n, v string) *Builder {
  	for i, a := range b.add {
  		if a.Name == n {
  			b.add[i].Value = v
  			return b
  		}
  	}
  	b.add = append(b.add, Label{Name: n, Value: v})
  
  	return b
  }
docs: // Set the name/value pair as a label.