code: func asSelfHeaders(c context.Context, uri string, opts *rpcOptions) (*oauth2.Token, map[string]string, error) {
  	cfg := getConfig(c)
  	if cfg == nil || cfg.AccessTokenProvider == nil {
  		return nil, nil, ErrNotConfigured
  	}
  	tok, err := cfg.AccessTokenProvider(c, opts.scopes)
  	return tok, nil, err
  }
docs: // asSelfHeaders returns a map of authentication headers to add to outbound
  // RPC requests done in AsSelf mode.
  //
  // This will be called by the transport layer on each request.