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/cron/station_status_test.go

@@ -9,6 +9,7 @@ import (
 	"github.com/go-redis/redis/v8"
 
 	"trip-planner/internal/cache"
+	"trip-planner/internal/metrics"
 	"trip-planner/internal/yandex"
 )
 
@@ -19,7 +20,7 @@ func newMockMonitor(id string, tripCount int, scheduleFunc func(context.Context,
 	monitor := &StationMonitor{
 		ID:           id,
 		Yandex:       yc,
-		Cache:        cache.NewCacheStore(rc),
+		Cache:        cache.NewCacheStore(rc, metrics.New()),
 		ScheduleFunc: scheduleFunc,
 	}