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/main.go

@@ -8,6 +8,7 @@ import (
 	"github.com/go-redis/redis/v8"
 
 	"trip-planner/internal/cache"
+	"trip-planner/internal/metrics"
 	"trip-planner/internal/routing"
 	"trip-planner/internal/yandex"
 )
@@ -15,9 +16,10 @@ import (
 func main() {
 	redisClient := initRedis()
 	router := routing.NewGraph()
-	yandexClient := yandex.NewClient("default-key")
+	m := metrics.New()
+	yandexClient := yandex.NewClient("default-key", yandex.WithMetrics(m))
 
-	handlerCtx := NewHandlerContext(redisClient, router, yandexClient)
+	handlerCtx := NewHandlerContext(redisClient, router, yandexClient, m)
 
 	// Cache warm-up: load city directory into Redis cache
 	// ensures the API functions correctly on cold start and after cache expiry
@@ -29,6 +31,7 @@ func main() {
 	http.HandleFunc("/v1/routes/", makeHandler(RouteGeoJSON, handlerCtx))
 	http.HandleFunc("/v1/stations/", makeHandler(StationStatus, handlerCtx))
 	http.HandleFunc("/internal/admin/stations/", makeHandler(AdminStationStatus, handlerCtx))
+	http.HandleFunc("/metrics", makeHandler(MetricsHandler, handlerCtx))
 
 	log.Println("Trip Planner API starting on :8080")
 	log.Fatal(http.ListenAndServe(":8080", nil))