Add server-side stats for decentralized learning

[Grim-bot]

In decentralized learning with secure aggregation, the helper server can keep track of who aggregates with whom. Thus, the server can easily collect statistics like the number of aggregation rounds each client has participated in.

It would be useful to display these, e.g. as a histogram. How does that work ?

[martinjaggi]

can copy the server stats from the federated case (hopefully can share the code somehow)