feat(middleware): export clientqueue_enqueued_total counter

Counts every request placed in a wait queue regardless of outcome, so the
total ever queued no longer has to be summed from queued, timeout and
canceled.

pkg/middleware/README.md

@@ -449,6 +449,7 @@ no per-client labels so cardinality stays bounded.
 | Metric | Type | Meaning |
 |---|---|---|
 | `clientqueue_requests_total{result}` | counter | `immediate`, `queued`, `rejected_full`, `timeout`, `canceled` |
+| `clientqueue_enqueued_total` | counter | requests ever placed in a wait queue, whatever happened next (ran, timed out, cancelled) |
 | `clientqueue_wait_seconds` | histogram | wait for a slot; 0 for requests that ran immediately |
 | `clientqueue_wait_max_seconds` | gauge | longest wait since process start |
 | `clientqueue_burst_size` | histogram | peak outstanding requests per client busy period |

pkg/middleware/clientqueue.go

@@ -70,6 +70,11 @@ var (
 		Help: "Requests currently waiting for a slot",
 	})
 
+	queueEnqueued = promauto.NewCounter(prometheus.CounterOpts{
+		Name: "clientqueue_enqueued_total",
+		Help: "Requests ever placed in a wait queue, whatever their outcome (ran, timed out or cancelled)",
+	})
+
 	queueWaitingClients = promauto.NewGauge(prometheus.GaugeOpts{
 		Name: "clientqueue_waiting_clients",
 		Help: "Clients currently with at least one request waiting for a slot",
@@ -286,6 +291,7 @@ func (q *ClientQueue) acquire(ctx context.Context, key string) error {
 	c.enter()
 	q.mu.Unlock()
 	queueDepth.Inc()
+	queueEnqueued.Inc()
 
 	timer := time.NewTimer(q.cfg.MaxWait)
 	defer timer.Stop()

pkg/middleware/clientqueue_test.go

@@ -279,6 +279,7 @@ func TestClientQueueMetrics(t *testing.T) {
 	q := newTestQueue(t, ClientQueueConfig{MaxConcurrent: 2})
 	imm0 := gaugeVal(t, queueRequests.WithLabelValues("immediate"))
 	que0 := gaugeVal(t, queueRequests.WithLabelValues("queued"))
+	enq0 := gaugeVal(t, queueEnqueued)
 	bc0, bs0 := histVal(t, queueBurst)
 	wc0, _ := histVal(t, queueWait)
 
@@ -318,6 +319,9 @@ func TestClientQueueMetrics(t *testing.T) {
 	if got := gaugeVal(t, queueRequests.WithLabelValues("immediate")) - imm0; got != 2 {
 		t.Errorf("immediate = %v, want 2", got)
 	}
+	if got := gaugeVal(t, queueEnqueued) - enq0; got != 3 {
+		t.Errorf("enqueued = %v, want 3", got)
+	}
 	if got := gaugeVal(t, queueRequests.WithLabelValues("queued")) - que0; got != 3 {
 		t.Errorf("queued = %v, want 3", got)
 	}
@@ -404,3 +408,27 @@ func TestClientQueueWaitingClientsGauge(t *testing.T) {
 	waitFor(0)
 	q.release("b")
 }
+
+func TestClientQueueEnqueuedCountsAllOutcomes(t *testing.T) {
+	q := newTestQueue(t, ClientQueueConfig{MaxConcurrent: 1, MaxQueue: 1, MaxWait: 50 * time.Millisecond})
+	base := gaugeVal(t, queueEnqueued)
+	if err := q.acquire(t.Context(), "k"); err != nil { // immediate: not enqueued
+		t.Fatal(err)
+	}
+	if err := q.acquire(t.Context(), "k"); !errors.Is(err, errQueueWait) { // enqueued, times out
+		t.Fatalf("err = %v, want timeout", err)
+	}
+	cctx, cancel := context.WithCancel(t.Context())
+	done := make(chan error, 1)
+	go func() { done <- q.acquire(cctx, "k") }() // enqueued, cancelled
+	time.Sleep(10 * time.Millisecond)
+	if err := q.acquire(t.Context(), "k"); !errors.Is(err, errQueueFull) { // rejected: not enqueued
+		t.Fatalf("err = %v, want queue full", err)
+	}
+	cancel()
+	<-done
+	q.release("k")
+	if got := gaugeVal(t, queueEnqueued) - base; got != 2 {
+		t.Fatalf("enqueued = %v, want 2 (timeout + cancel; not immediate or rejected)", got)
+	}
+}