Number of datagrams received.

Depending on batching settings, a single datagram may contain one or more messages, or a fragment of a larger message. Larger messages are split into fragment sizes limited by the transport's configuration option transport_xxxx_datagram_max_size. This method returns a value from the first set of statistics encapsulated within this object and would normally be used when the object has been created using a specific source to instantiate the class.