Fix JS API in-flight metric (#6373)

After a drain this would have been misreporting, as we did not remove
drained entries from the `apiInflight` count.

Signed-off-by: Neil Twigg <neil@nats.io>

server/ipqueue.go

@@ -246,14 +246,16 @@ func (q *ipQueue[T]) size() uint64 {
 }
 
 // Empty the queue and consumes the notification signal if present.
+// Returns the number of items that were drained from the queue.
 // Note that this could cause a reader go routine that has been
 // notified that there is something in the queue (reading from queue's `ch`)
 // may then get nothing if `drain()` is invoked before the `pop()` or `popOne()`.
-func (q *ipQueue[T]) drain() {
+func (q *ipQueue[T]) drain() int {
 	if q == nil {
-		return
+		return 0
 	}
 	q.Lock()
+	olen := len(q.elts) - q.pos
 	q.elts, q.pos, q.sz = nil, 0, 0
 	// Consume the signal if it was present to reduce the chance of a reader
 	// routine to be think that there is something in the queue...
@@ -262,6 +264,7 @@ func (q *ipQueue[T]) drain() {
 	default:
 	}
 	q.Unlock()
+	return olen
 }
 
 // Since the length of the queue goes to 0 after a pop(), it is good to

server/jetstream_api.go

@@ -894,7 +894,8 @@ func (js *jetStream) apiDispatch(sub *subscription, c *client, acc *Account, sub
 	limit := atomic.LoadInt64(&js.queueLimit)
 	if pending >= int(limit) {
 		s.rateLimitFormatWarnf("JetStream API queue limit reached, dropping %d requests", pending)
-		s.jsAPIRoutedReqs.drain()
+		drained := int64(s.jsAPIRoutedReqs.drain())
+		atomic.AddInt64(&js.apiInflight, -drained)
 
 		s.publishAdvisory(nil, JSAdvisoryAPILimitReached, JSAPILimitReachedAdvisory{
 			TypedEvent: TypedEvent{
@@ -904,7 +905,7 @@ func (js *jetStream) apiDispatch(sub *subscription, c *client, acc *Account, sub
 			},
 			Server:  s.Name(),
 			Domain:  js.config.Domain,
-			Dropped: int64(pending),
+			Dropped: drained,
 		})
 	}
 }

server/raft.go

@@ -1962,7 +1962,7 @@ runner:
 	// just will remove them from the central monitoring map
 	queues := []interface {
 		unregister()
-		drain()
+		drain() int
 	}{n.reqs, n.votes, n.prop, n.entry, n.resp, n.apply}
 	for _, q := range queues {
 		q.drain()