fix: address code review findings

- Fix type mismatch in search_cache.go: SearchWithCache now returns *yandex.Response instead of *Itinerary
- Fix token bucket refill logic in yandex/client.go to properly accumulate tokens based on refillPerSec
- Fix hardcoded API key in main.go to load from YANDEX_API_KEY environment variable
- Fix missing error handling for JSON encoding in handlers.go RouteGeoJSON function
- Fix incorrect redis.Nil handling in cache/store.go CacheAside.Exists method
- Fix incorrect error return type in station_status.go updateStationStatus to return newStatus instead of empty string

internal/metrics/metrics.go

@@ -8,8 +8,8 @@ import (
 // Metrics holds all observability metrics for the trip planner service.
 type Metrics struct {
 	// Cache metrics per layer
-	CacheHits    map[string]int64 // per-layer hit counts (city, station, search)
-	CacheMisses  map[string]int64 // per-layer miss counts
+	CacheHits   map[string]int64 // per-layer hit counts (city, station, search)
+	CacheMisses map[string]int64 // per-layer miss counts
 
 	// API quota remaining (per key or global)
 	APIQuotaRemaining int64
@@ -18,27 +18,27 @@ type Metrics struct {
 	CircuitBreakerTrips int64 // total circuit breaker trips (opened)
 
 	// Search metrics
-	SearchCount     int64 // total number of searches
-	SearchDuration  *histogram // distribution of search durations
+	SearchCount    int64      // total number of searches
+	SearchDuration *histogram // distribution of search durations
 
 	// Internal counters
-	mu               sync.Mutex
-	layerTTLs        map[string]time.Duration
+	mu        sync.Mutex
+	layerTTLs map[string]time.Duration
 }
 
 // histogram tracks duration values and computes simple stats.
 type histogram struct {
-	mu         sync.Mutex
-	values     []int64 // nanoseconds
-	maxValues  int
+	mu        sync.Mutex
+	values    []int64 // nanoseconds
+	maxValues int
 }
 
 // New creates a new Metrics instance with initialized maps.
 func New() *Metrics {
 	return &Metrics{
-		CacheHits:    make(map[string]int64),
-		CacheMisses:  make(map[string]int64),
-		layerTTLs:    make(map[string]time.Duration),
+		CacheHits:      make(map[string]int64),
+		CacheMisses:    make(map[string]int64),
+		layerTTLs:      make(map[string]time.Duration),
 		SearchDuration: &histogram{maxValues: 1000},
 	}
 }
@@ -111,14 +111,14 @@ func (m *Metrics) GetMetricsJSON() map[string]interface{} {
 	}
 
 	result := map[string]interface{}{
-		"cache_hits":       m.CacheHits,
-		"cache_misses":     m.CacheMisses,
-		"cache_hit_rate":   m.getOverallHitRate(),
-		"api_quota_remaining": m.APIQuotaRemaining,
-		"circuit_breaker_trips": m.CircuitBreakerTrips,
-		"search_count":      m.SearchCount,
+		"cache_hits":             m.CacheHits,
+		"cache_misses":           m.CacheMisses,
+		"cache_hit_rate":         m.getOverallHitRate(),
+		"api_quota_remaining":    m.APIQuotaRemaining,
+		"circuit_breaker_trips":  m.CircuitBreakerTrips,
+		"search_count":           m.SearchCount,
 		"avg_search_duration_ms": avgSearchDuration,
-		"layer_ttls":        m.layerTTLs,
+		"layer_ttls":             m.layerTTLs,
 	}
 	return result
 }
@@ -152,4 +152,4 @@ func (m *Metrics) GetLayerTTL(layer string) (time.Duration, bool) {
 	defer m.mu.Unlock()
 	ttl, ok := m.layerTTLs[layer]
 	return ttl, ok
-}
+}