A protocol called to track resources.

This is invoked by FLAME from FLAME.track_resources/3, which is invoked when the :track_resources option is set to true.

Sometimes we may want to allocate long lived resources in a FLAME but, because FLAME nodes are temporary, the node would terminate shortly after. The :track_resources option tells FLAME to look for resources which implement the FLAME.Trackable protocol. Those resources can then spawn PIDs in the remote node and tell FLAME to track them. Once all PIDs terminate, the FLAME will terminate too.

Implementations of the protocol will receive the data type, a list of pids as acc, and the node. It must return the updated data type and an updated list of pids. If you need to traverse recursively, you may call FLAME.track_resources/3.