discovery/gce/gce.go
func (c *SDConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
	*c = DefaultSDConfig
	type plain SDConfig
	err := unmarshal((*plain)(c))
	if err != nil {
		return err
	}
	if c.Project == "" {
		return errors.New("GCE SD configuration requires a project")
	}
	if c.Zone == "" {
		return errors.New("GCE SD configuration requires a zone")
	}
	return nil
}
func (c *SDConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
	*c = DefaultSDConfig
	type plain SDConfig
	err := unmarshal((*plain)(c))
	if err != nil {
		return err
	}
	if c.Project == "" {
		return errors.New("GCE SD configuration requires a project")
	}
	if c.Zone == "" {
		return errors.New("GCE SD configuration requires a zone")
	}
	return nil
}
// UnmarshalYAML implements the yaml.Unmarshaler interface.