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.go

@@ -5,10 +5,12 @@ import (
 	"fmt"
 	"net/http"
 	"strings"
+	"time"
 
 	"github.com/go-redis/redis/v8"
 
 	"trip-planner/internal/cache"
+	"trip-planner/internal/metrics"
 	"trip-planner/internal/routing"
 	"trip-planner/internal/storage"
 	"trip-planner/internal/yandex"
@@ -16,12 +18,14 @@ import (
 
 // HandlerContext holds the dependencies for API handlers.
 type HandlerContext struct {
-	Cache       cache.Cache
-	Redis       *redis.Client
-	Router      *routing.Graph
-	Yandex      *yandex.Client
-	SearchCache *routing.SearchCacheService
-	Preferences *cache.Preferences
+	Cache         cache.Cache
+	Redis         *redis.Client
+	Router        *routing.Graph
+	Yandex        *yandex.Client
+	SearchCache   *routing.SearchCacheService
+	Preferences   *cache.Preferences
+	Metrics       *metrics.Metrics
+	SearchStart   time.Time
 }
 
 // stationStatusResponse represents the response for station status.
@@ -183,6 +187,9 @@ func RouteSearch(hc *HandlerContext, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Record search start time
+	start := time.Now()
+
 	// Read ranking mode from query parameters (for UI controls)
 	rankingMode := r.URL.Query().Get("ranking_mode")
 
@@ -201,6 +208,10 @@ func RouteSearch(hc *HandlerContext, w http.ResponseWriter, r *http.Request) {
 	// Run Pareto-optimal route search using the graph
 	results := hc.Router.FindRoutesPareto(req.FromCityID, req.ToCityID, opts)
 
+	// Record search duration
+	duration := time.Since(start).Nanoseconds()
+	hc.Metrics.RecordSearch(duration)
+
 	// Build response routes
 	routeResponses := make([]routeSearchRoute, 0, len(results))
 	for _, route := range results {
@@ -598,15 +609,23 @@ func AddSearchHistory(hc *HandlerContext, w http.ResponseWriter, r *http.Request
 	})
 }
 
+// metricsHandler handles GET /metrics and returns all observability metrics as JSON.
+func MetricsHandler(hc *HandlerContext, w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(hc.Metrics.GetMetricsJSON())
+}
+
 // NewHandlerContext creates a new HandlerContext with initialized services.
-func NewHandlerContext(redisClient *redis.Client, router *routing.Graph, yandex *yandex.Client) *HandlerContext {
-	cacheStore := cache.NewCacheStore(redisClient)
+func NewHandlerContext(redisClient *redis.Client, router *routing.Graph, yandex *yandex.Client, m *metrics.Metrics) *HandlerContext {
+	cacheStore := cache.NewCacheStore(redisClient, m)
 	return &HandlerContext{
-		Cache:       cacheStore,
-		Redis:       redisClient,
-		Router:      router,
-		Yandex:      yandex,
-		SearchCache: routing.NewSearchCacheService(cache.NewCacheAside(cacheStore), yandex),
+		Cache:      cacheStore,
+		Redis:      redisClient,
+		Router:     router,
+		Yandex:     yandex,
+		SearchCache: routing.NewSearchCacheService(cacheStore, yandex, m),
 		Preferences: cache.NewPreferences(cacheStore),
+		Metrics:    m,
+		SearchStart: time.Now(),
 	}
 }