fair_queue: track the total capacity of queued requests

Adds a member variable which tracks the summed capacity of
all requests waiting in the queue.
This is a piece of data which might be valuable to the IO scheduler.
We make use of it in later patches in the series.

include/seastar/core/fair_queue.hh

@@ -350,6 +350,8 @@ private:
     };
 
     std::optional<pending> _pending;
+    // Total capacity of all requests waiting in the queue.
+    capacity_t _queued_capacity = 0;
 
     void push_priority_class(priority_class_data& pc) noexcept;
     void push_priority_class_from_idle(priority_class_data& pc) noexcept;

src/core/fair_queue.cc

@@ -297,12 +297,14 @@ void fair_queue::queue(class_id id, fair_queue_entry& ent) noexcept {
         push_priority_class_from_idle(pc);
     }
     pc._queue.push_back(ent);
+    _queued_capacity += ent.capacity();
 }
 
 void fair_queue::notify_request_finished(fair_queue_entry::capacity_t cap) noexcept {
 }
 
 void fair_queue::notify_request_cancelled(fair_queue_entry& ent) noexcept {
+    _queued_capacity -= ent._capacity;
     ent._capacity = 0;
 }
 
@@ -375,6 +377,7 @@ void fair_queue::dispatch_requests(std::function<void(fair_queue_entry&)> cb) {
         h._accumulated += req_cost;
         h._pure_accumulated += req_cap;
         dispatched += req_cap;
+        _queued_capacity -= req_cap;
 
         cb(req);