code: func NewDNSProvider() (*DNSProvider, error) {
  	// Use a service account file if specified via environment variable.
  	if saKey := env.GetOrFile("GCE_SERVICE_ACCOUNT"); len(saKey) > 0 {
  		return NewDNSProviderServiceAccountKey([]byte(saKey))
  	}
  
  	// Use default credentials.
  	project := env.GetOrDefaultString("GCE_PROJECT", "")
  	return NewDNSProviderCredentials(project)
  }
docs: // NewDNSProvider returns a DNSProvider instance configured for Google Cloud DNS.
  // Project name must be passed in the environment variable: GCE_PROJECT.
  // A Service Account can be passed in the environment variable: GCE_SERVICE_ACCOUNT
  // or by specifying the keyfile location: GCE_SERVICE_ACCOUNT_FILE