code: func (c *bootTimeCollector) Update(ch chan<- prometheus.Metric) error {
  	v, err := c.boottime.Value()
  	if err != nil {
  		return err
  	}
  
  	ch <- prometheus.MustNewConstMetric(
  		prometheus.NewDesc(
  			prometheus.BuildFQName(namespace, "", c.boottime.name),
  			c.boottime.description,
  			nil, nil,
  		), prometheus.GaugeValue, v)
  
  	return nil
  }
docs: // Update pushes boot time onto ch