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

internal/cache/store_test.go

@@ -5,6 +5,8 @@ import (
 	"testing"
 
 	"github.com/go-redis/redis/v8"
+
+	"trip-planner/internal/metrics"
 )
 
 // TestCacheGetSet tests basic Get and Set operations.
@@ -12,7 +14,7 @@ func TestCacheGetSet(t *testing.T) {
 	ctx := context.Background()
 	client := NewRedisClient(redis.NewClient(&redis.Options{
 		Addr: "localhost:6379",
-	}))
+	}), metrics.New())
 
 	// Test Set
 	key := &CacheKey{Kind: "city", Code: "c146"}
@@ -81,7 +83,7 @@ func TestCacheAsideGetOrSet(t *testing.T) {
 	ctx := context.Background()
 	client := NewRedisClient(redis.NewClient(&redis.Options{
 		Addr: "localhost:6379",
-	}))
+	}), metrics.New())
 
 	fetchCallCount := 0
 	fetchFunc := func() ([]byte, error) {
@@ -91,7 +93,7 @@ func TestCacheAsideGetOrSet(t *testing.T) {
 
 	// First call: cache miss, should fetch from backend
 	key := &CacheKey{Kind: "station", Code: "s9600213"}
-	data, err := NewCacheAside(client).GetOrSetFuncPattern(ctx, key, fetchFunc, CityTTL)
+	data, err := NewCacheAside(client, metrics.New()).GetOrSetFuncPattern(ctx, key, fetchFunc, CityTTL)
 	if err != nil {
 		t.Fatalf("expected no error on cache miss, got: %v", err)
 	}
@@ -104,7 +106,7 @@ func TestCacheAsideGetOrSet(t *testing.T) {
 
 	// Second call: cache hit, should not fetch from backend
 	fetchCallCount = 0
-	data, err = NewCacheAside(client).GetOrSetFuncPattern(ctx, key, fetchFunc, CityTTL)
+	data, err = NewCacheAside(client, metrics.New()).GetOrSetFuncPattern(ctx, key, fetchFunc, CityTTL)
 	if err != nil {
 		t.Fatalf("expected no error on cache hit, got: %v", err)
 	}
@@ -121,7 +123,7 @@ func TestCacheAsideGetCity(t *testing.T) {
 	ctx := context.Background()
 	client := NewRedisClient(redis.NewClient(&redis.Options{
 		Addr: "localhost:6379",
-	}))
+	}), metrics.New())
 
 	fetchCallCount := 0
 	fetchFunc := func() ([]byte, error) {
@@ -130,7 +132,7 @@ func TestCacheAsideGetCity(t *testing.T) {
 	}
 
 	key := &CacheKey{Kind: "city", Code: "c146"}
-	data, err := NewCacheAside(client).GetCity(ctx, key, fetchFunc)
+	data, err := NewCacheAside(client, metrics.New()).GetCity(ctx, key, fetchFunc)
 	if err != nil {
 		t.Fatalf("expected no error on cache miss for city, got: %v", err)
 	}
@@ -143,10 +145,13 @@ func TestCacheAsideGetCity(t *testing.T) {
 
 	// Second call: cache hit
 	fetchCallCount = 0
-	data, err = NewCacheAside(client).GetCity(ctx, key, fetchFunc)
+	data, err = NewCacheAside(client, metrics.New()).GetCity(ctx, key, fetchFunc)
 	if err != nil {
 		t.Fatalf("expected no error on cache hit for city, got: %v", err)
 	}
+	if string(data) != `{"code":"c146","title":"Simferopol"}` {
+		t.Errorf("expected %s, got %s", `{"code":"c146","title":"Simferopol"}`, string(data))
+	}
 	if fetchCallCount != 0 {
 		t.Errorf("expected 0 fetch calls on cache hit, got %d", fetchCallCount)
 	}
@@ -157,7 +162,7 @@ func TestCacheAsideGetSearch(t *testing.T) {
 	ctx := context.Background()
 	client := NewRedisClient(redis.NewClient(&redis.Options{
 		Addr: "localhost:6379",
-	}))
+	}), metrics.New())
 
 	fetchNearTerm := func() ([]byte, error) {
 		return []byte(`{"near_term":true}`), nil
@@ -172,7 +177,7 @@ func TestCacheAsideGetSearch(t *testing.T) {
 	farKey := &CacheKey{Kind: "search", From: "c146", To: "c213", Date: "2026-09-15"}
 
 	// Near-term: should use SearchNearTermTTL (3 hours)
-	data, err := NewCacheAside(client).GetSearch(ctx, nearKey, fetchNearTerm, false)
+	data, err := NewCacheAside(client, metrics.New()).GetSearch(ctx, nearKey, fetchNearTerm, false)
 	if err != nil {
 		t.Fatalf("expected no error on near-term search cache miss, got: %v", err)
 	}
@@ -181,7 +186,7 @@ func TestCacheAsideGetSearch(t *testing.T) {
 	}
 
 	// Far-term: should use SearchFarTermTTL (7 days)
-	data, err = NewCacheAside(client).GetSearch(ctx, farKey, fetchFarTerm, true)
+	data, err = NewCacheAside(client, metrics.New()).GetSearch(ctx, farKey, fetchFarTerm, true)
 	if err != nil {
 		t.Fatalf("expected no error on far-term search cache miss, got: %v", err)
 	}
@@ -195,7 +200,7 @@ func TestCacheInvalidate(t *testing.T) {
 	ctx := context.Background()
 	client := NewRedisClient(redis.NewClient(&redis.Options{
 		Addr: "localhost:6379",
-	}))
+	}), metrics.New())
 
 	// Set up some keys
 	cityKey := &CacheKey{Kind: "city", Code: "c146"}
@@ -208,20 +213,20 @@ func TestCacheInvalidate(t *testing.T) {
 	client.Set(ctx, searchKey, []byte(`{"search":true}`), SearchNearTermTTL)
 
 	// Invalidate city
-	err := NewCacheAside(client).InvalidateCity(ctx, cityKey)
+	err := NewCacheAside(client, metrics.New()).InvalidateCity(ctx, cityKey)
 	if err != nil {
 		t.Fatalf("expected no error invalidating city, got: %v", err)
 	}
 
 	// Invalidate station
-	err = NewCacheAside(client).InvalidateStation(ctx, stationKey)
+	err = NewCacheAside(client, metrics.New()).InvalidateStation(ctx, stationKey)
 	if err != nil {
 		t.Fatalf("expected no error invalidating station, got: %v", err)
 	}
 
 	// Invalidate search
-	err = NewCacheAside(client).InvalidateSearch(ctx, searchKey)
+	err = NewCacheAside(client, metrics.New()).InvalidateSearch(ctx, searchKey)
 	if err != nil {
 		t.Fatalf("expected no error invalidating search, got: %v", err)
 	}
-}
+}