[hw-7] add metrics, tracing

cart/internal/infra/http/metrics/repository.go

@@ -0,0 +1,16 @@
+package metrics
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var inMemoryObjectsGauge = promauto.NewGauge(prometheus.GaugeOpts{
+	Namespace: "app",
+	Name:      "inmemory_repo_objects",
+	Help:      "Current in-memory repository size",
+})
+
+func SetInMemoryObjects(n int) {
+	inMemoryObjectsGauge.Set(float64(n))
+}