code: func (p *ProviderAggregator) AddProvider(provider provider.Provider) error {
  	err := provider.Init()
  	if err != nil {
  		return err
  	}
  
  	if fileProvider, ok := provider.(*file.Provider); ok {
  		p.fileProvider = fileProvider
  	} else {
  		p.providers = append(p.providers, provider)
  	}
  	return nil
  }
docs: // AddProvider adds a provider in the providers map.