code: func (s *serviceCache) ListKeys() []string {
  	s.mu.Lock()
  	defer s.mu.Unlock()
  	keys := make([]string, 0, len(s.serviceMap))
  	for k := range s.serviceMap {
  		keys = append(keys, k)
  	}
  	return keys
  }
docs: // ListKeys implements the interface required by DeltaFIFO to list the keys we
  // already know about.