A graceful-shutdown coordinator for a zRPC server (← go-zero's
proc.AddShutdownListener + gRPC's GracefulStop): once shutdown is
initiated the server stops admitting new calls, but calls already in flight are
allowed to run to completion. A call brackets its work between begin_call and
end_call; begin_call returns false when the server is shutting down, which
the transport surfaces as Unavailable — exactly the status a client sees once
a server has stopped listening. The server is fully drained once shutdown has
been initiated and no calls remain in flight.
The counter is plain mutable state, which is safe under moonbitlang/async's
cooperative single-threaded scheduling: begin_call/end_call never yield, so
the count is only observed at await points between them.