[#1653] qos: Add metrics

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

internal/qos/metrics.go

@@ -0,0 +1,31 @@
+package qos
+
+import "sync/atomic"
+
+type Metrics interface {
+	SetOperationTagCounters(shardID, operation, tag string, pending, inProgress, completed, resourceExhausted uint64)
+	Close(shardID string)
+}
+
+var _ Metrics = (*noopMetrics)(nil)
+
+type noopMetrics struct{}
+
+func (n *noopMetrics) SetOperationTagCounters(string, string, string, uint64, uint64, uint64, uint64) {
+}
+
+func (n *noopMetrics) Close(string) {}
+
+// stat presents limiter statistics cumulative counters.
+//
+// Each operation changes its status as follows: `pending` -> `in_progress` -> `completed` or `resource_exhausted`.
+type stat struct {
+	completed         atomic.Uint64
+	pending           atomic.Uint64
+	resourceExhausted atomic.Uint64
+	inProgress        atomic.Uint64
+}
+
+type metricsHolder struct {
+	metrics Metrics
+}