gserviceaccount/opts.go
func Expiration(expiration time.Duration) Option {
	return func(gsa *GServiceAccount) error {
		gsa.expiration = expiration
		return nil
	}
}
func Expiration(expiration time.Duration) Option {
	return func(gsa *GServiceAccount) error {
		gsa.expiration = expiration
		return nil
	}
}
// Expiration is a GServiceAccount option to set a expiration limit for tokens
// generated from the token source.