code: func (m *metadataTracker) Counts(dev protocol.DeviceID, flag uint32) Counts {
  	if bits.OnesCount32(flag) > 1 {
  		panic("incorrect usage: set at most one bit in flag")
  	}
  
  	m.mut.RLock()
  	defer m.mut.RUnlock()
  
  	idx, ok := m.indexes[metaKey{dev, flag}]
  	if !ok {
  		return Counts{}
  	}
  
  	return m.counts.Counts[idx]
  }
docs: // Counts returns the counts for the given device ID and flag. `flag` should
  // be zero or have exactly one bit set.