code: func (m *metadataTracker) Unmarshal(bs []byte) error {
  	if err := m.counts.Unmarshal(bs); err != nil {
  		return err
  	}
  
  	// Initialize the index map
  	for i, c := range m.counts.Counts {
  		m.indexes[metaKey{protocol.DeviceIDFromBytes(c.DeviceID), c.LocalFlags}] = i
  	}
  	return nil
  }
docs: // Unmarshal loads a metadataTracker from the corresponding protobuf
  // representation