docs: def WriteClientStartupInfo(self, client_id, startup_info):
      """Writes a new client startup record."""
      if client_id not in self.metadatas:
        raise db.UnknownClientError(client_id)
  
      ts = rdfvalue.RDFDatetime.Now()
      self.metadatas[client_id]["startup_info_timestamp"] = ts
      history = self.startup_history.setdefault(client_id, {})
      history[ts] = startup_info.SerializeToString()