feat: implement observability and metrics (Task 22)

- Add metrics package tracking cache hit-rate, API quota, circuit breaker trips, search time
- Integrate metrics with cache layer, yandex client, and API handlers
- Add /metrics HTTP endpoint for Prometheus-compatible metrics exposure
- Write tests for metrics functionality across cache, yandex, and API handlers
- Update test files to support new metrics infrastructure

cmd/api/handlers_test.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/go-redis/redis/v8"
 
+	"trip-planner/internal/metrics"
 	"trip-planner/internal/airports"
 	"trip-planner/internal/routing"
 	"trip-planner/internal/storage"
@@ -54,7 +55,7 @@ func newMockHandlerContext() *HandlerContext {
 	// Create Yandex client
 	yandexClient := yandex.NewClient("test-key")
 
-	return NewHandlerContext(redisClient, router, yandexClient)
+	return NewHandlerContext(redisClient, router, yandexClient, metrics.New())
 }
 
 func TestHandlerCityAutocomplete(t *testing.T) {