[broadcast] use subscribe metric and batch increment

[stevencartavia]

Update the metric from get to subscribe in cleanup_waiters and increment it in batches

[cursor]

Bug: Metric count truncation on wide platforms

dropped_count is a usize but gets cast with dropped_count as u64 for subscribe.inc_by. On platforms where usize is wider than 64 bits (or with extremely large waiter lists), this cast truncates and silently undercounts dropped waiters in metrics.