code: func (id ObjectId) Counter() int32 {
  	b := id.byteSlice(9, 12)
  	// Counter is stored as big-endian 3-byte value
  	return int32(uint32(b[0])<<16 | uint32(b[1])<<8 | uint32(b[2]))
  }
docs: // Counter returns the incrementing value part of the id.
  // It's a runtime error to call this method with an invalid id.