code: func asActorHeaders(c context.Context, uri string, opts *rpcOptions) (*oauth2.Token, map[string]string, error) {
  	mintTokenCall := MintAccessTokenForServiceAccount
  	if opts.rpcMocks != nil && opts.rpcMocks.MintAccessTokenForServiceAccount != nil {
  		mintTokenCall = opts.rpcMocks.MintAccessTokenForServiceAccount
  	}
  	oauthTok, err := mintTokenCall(c, MintAccessTokenParams{
  		ServiceAccount: opts.serviceAccount,
  		Scopes:         opts.scopes,
  		MinTTL:         2 * time.Minute,
  	})
  	return oauthTok, nil, err
  }
docs: // asActorHeaders returns a map of authentication headers to add to outbound
  // RPC requests done in AsActor mode.
  //
  // This will be called by the transport layer on each request.