[hw-7] add metrics, tracing

cart/internal/domain/repository/in_memory_repository.go

@@ -6,6 +6,7 @@ import (
 
 	"route256/cart/internal/domain/entity"
 	"route256/cart/internal/domain/model"
+	"route256/cart/internal/infra/http/metrics"
 )
 
 type storage = map[entity.UID]*entity.Cart
@@ -34,6 +35,7 @@ func (r *InMemoryRepository) AddItem(_ context.Context, userID entity.UID, item
 		}
 
 		r.storage[userID] = cart
+		metrics.SetInMemoryObjects(len(r.storage))
 	}
 
 	if _, ok := cart.ItemCount[item.Product.Sku]; !ok {
@@ -112,6 +114,7 @@ func (r *InMemoryRepository) DeleteItemsByUserID(_ context.Context, userID entit
 	_, ok := r.storage[userID]
 	if ok {
 		delete(r.storage, userID)
+		metrics.SetInMemoryObjects(len(r.storage))
 	}
 
 	return nil