Metrics for the background decide processor. backlog (decided - processed) is the key signal:
sustained growth means staging tables accumulate (no data lost, but disk grows).
How many new-protocol decides to wait for before tearing down the legacy
consensus stack (tasks + network). Once the coordinator has decided even a
single leaf the cutover boundary is final and all consensus traffic runs
on the coordinator’s network; the margin only gives slightly-lagging peers
a window to finish crossing the boundary with legacy help.
Turns persisted decided leaves into query-service decide events and GCs processed data.
Decoupled from handle_events so slow ingestion/GC can’t stall (or drop) consensus events;
cursor-driven, so it can lag without losing data.
Latest decided view and its (optional) deciding QC, sent from the event loop to the background
decide processor. None is the initial/no-op value of the watch channel.