feat: add metrics, observability, test fixtures, and scaling playbook artifacts

backend/chat/src/metrics/metrics_test.go

@@ -54,6 +54,10 @@ func TestHandleNotFound(t *testing.T) {
 }
 
 func TestHandleMetrics(t *testing.T) {
+	chatRequestDurationHistogram = newRequestDurationHistogram(requestDurationBuckets)
+	ObserveRequestDuration(100 * time.Millisecond)
+	ObserveRequestDuration(750 * time.Millisecond)
+
 	gormDB, err := gorm.Open(sqlite.Open(":memory:"), &gorm.Config{})
 	if err != nil {
 		t.Fatalf("failed to open test DB: %v", err)
@@ -89,8 +93,12 @@ func TestHandleMetrics(t *testing.T) {
 		"linearvoid_chat_redis_enabled",
 		"linearvoid_chat_redis_up",
 		"linearvoid_chat_redis_connected_clients",
+		"linearvoid_chat_redis_memory_used_bytes",
+		"linearvoid_chat_redis_memory_max_bytes",
+		"linearvoid_chat_redis_memory_usage_ratio",
 		"linearvoid_chat_admin_sessions_active",
 		"linearvoid_chat_build_info",
+		"linearvoid_chat_request_duration_seconds",
 	}
 
 	for _, metric := range requiredMetrics {
@@ -117,6 +125,12 @@ func TestHandleMetrics(t *testing.T) {
 	if !strings.Contains(body, "linearvoid_chat_redis_enabled 0") {
 		t.Error("redis_enabled should be 0 (nil redis)")
 	}
+	if !strings.Contains(body, "linearvoid_chat_request_duration_seconds_bucket{le=\"1\"} 2") {
+		t.Error("request duration histogram should include both observed samples by the 1s bucket")
+	}
+	if !strings.Contains(body, "linearvoid_chat_request_duration_seconds_count 2") {
+		t.Error("request duration count should be 2")
+	}
 }
 
 func TestHandleMetrics_WrongMethod(t *testing.T) {
@@ -132,6 +146,9 @@ func TestHandleMetrics_WrongMethod(t *testing.T) {
 }
 
 func TestHandleMetrics_WithRedis(t *testing.T) {
+	chatRequestDurationHistogram = newRequestDurationHistogram(requestDurationBuckets)
+	ObserveRequestDuration(250 * time.Millisecond)
+
 	gormDB, _ := gorm.Open(sqlite.Open(":memory:"), &gorm.Config{})
 	gormDB.AutoMigrate(&db.Message{}, &db.ModerationLog{})
 
@@ -160,6 +177,15 @@ func TestHandleMetrics_WithRedis(t *testing.T) {
 	if !strings.Contains(body, "linearvoid_chat_redis_connected_clients 5") {
 		t.Error("redis_connected_clients should be 5")
 	}
+	if !strings.Contains(body, "linearvoid_chat_redis_memory_used_bytes 1048576") {
+		t.Error("redis_memory_used_bytes should be 1048576")
+	}
+	if !strings.Contains(body, "linearvoid_chat_redis_memory_usage_ratio 1") {
+		t.Error("redis_memory_usage_ratio should be 1 when maxmemory is unavailable or equal to used memory in tests")
+	}
+	if !strings.Contains(body, "linearvoid_chat_request_duration_seconds_count 1") {
+		t.Error("request duration count should be 1")
+	}
 }
 
 // metricsMockRedis is a minimal mock for metrics tests.
@@ -185,6 +211,6 @@ func (m *metricsMockRedis) Scan(ctx context.Context, cursor uint64, match string
 	return nil, 0, nil
 }
 func (m *metricsMockRedis) Info(ctx context.Context, sections ...string) (string, error) {
-	return "connected_clients:5\nused_memory_human:1M\n", nil
+	return "connected_clients:5\nused_memory_human:1M\nmaxmemory_human:1M\n", nil
 }
 func (m *metricsMockRedis) Close() error { return nil }