Create a new zero-initialized/empty entry in the received_datagrams dict when a new session is created using self._statistics.received_datagram.setdefault(...) and pass a reference to it to the session constructor. Let the session update the counters while it exists. When the session is terminated, the corresponding entry in the received_datagram dict is not to be removed.
To close: https://github.com/OpenCyphal/pycyphal/issues/279