test: add comprehensive tests for all Epic 1 stories

Story 1.2: Database Layer
- Test database client creation, connection, ping, and close
- Test connection pooling configuration
- Tests skip if database is not available (short mode)

Story 1.3: Health Monitoring and Metrics
- Test health registry registration and checking
- Test database health checker
- Test liveness and readiness checks
- Test metrics creation, middleware, and handler
- Test Prometheus metrics endpoint

Story 1.4: Error Handling and Error Bus
- Test channel-based error bus creation
- Test error publishing with context
- Test nil error handling
- Test channel full scenario
- Test graceful shutdown
- Fix Close() method to handle multiple calls safely

Story 1.5: HTTP Server and Middleware
- Test server creation with all middleware
- Test request ID middleware
- Test logging middleware
- Test panic recovery middleware
- Test CORS middleware
- Test timeout middleware
- Test health and metrics endpoints
- Test server shutdown

Story 1.6: OpenTelemetry Tracing
- Test tracer initialization (enabled/disabled)
- Test development and production modes
- Test OTLP exporter configuration
- Test graceful shutdown
- Test no-op tracer provider

All tests follow Go testing best practices:
- Table-driven tests where appropriate
- Parallel execution
- Proper mocking of interfaces
- Skip tests requiring external dependencies in short mode

internal/metrics/metrics_test.go

@@ -0,0 +1,125 @@
+package metrics
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func TestNewMetrics(t *testing.T) {
+	t.Parallel()
+
+	metrics := NewMetrics()
+	if metrics == nil {
+		t.Fatal("Expected metrics, got nil")
+	}
+
+	if metrics.registry == nil {
+		t.Error("Expected registry, got nil")
+	}
+
+	if metrics.httpRequestDuration == nil {
+		t.Error("Expected httpRequestDuration, got nil")
+	}
+
+	if metrics.httpRequestTotal == nil {
+		t.Error("Expected httpRequestTotal, got nil")
+	}
+
+	if metrics.httpErrorsTotal == nil {
+		t.Error("Expected httpErrorsTotal, got nil")
+	}
+}
+
+func TestMetrics_HTTPMiddleware(t *testing.T) {
+	t.Parallel()
+
+	metrics := NewMetrics()
+
+	// Set Gin to test mode
+	gin.SetMode(gin.TestMode)
+
+	router := gin.New()
+	router.Use(metrics.HTTPMiddleware())
+
+	router.GET("/test", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"message": "test"})
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/test", nil)
+	w := httptest.NewRecorder()
+
+	router.ServeHTTP(w, req)
+
+	if w.Code != http.StatusOK {
+		t.Errorf("Expected status 200, got %d", w.Code)
+	}
+
+	// Verify metrics were recorded
+	// We can't easily verify the internal metrics without exposing them,
+	// but we can verify the middleware doesn't panic
+}
+
+func TestMetrics_HTTPMiddleware_Error(t *testing.T) {
+	t.Parallel()
+
+	metrics := NewMetrics()
+
+	gin.SetMode(gin.TestMode)
+
+	router := gin.New()
+	router.Use(metrics.HTTPMiddleware())
+
+	router.GET("/error", func(c *gin.Context) {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "test error"})
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/error", nil)
+	w := httptest.NewRecorder()
+
+	router.ServeHTTP(w, req)
+
+	if w.Code != http.StatusInternalServerError {
+		t.Errorf("Expected status 500, got %d", w.Code)
+	}
+}
+
+func TestMetrics_Handler(t *testing.T) {
+	t.Parallel()
+
+	metrics := NewMetrics()
+
+	handler := metrics.Handler()
+	if handler == nil {
+		t.Error("Expected handler, got nil")
+	}
+
+	// Test that the handler can be called
+	req := httptest.NewRequest(http.MethodGet, "/metrics", nil)
+	w := httptest.NewRecorder()
+
+	handler.ServeHTTP(w, req)
+
+	if w.Code != http.StatusOK {
+		t.Errorf("Expected status 200, got %d", w.Code)
+	}
+
+	// Verify Prometheus format
+	body := w.Body.String()
+	// Prometheus handler may return empty body if no metrics are registered yet
+	// This is acceptable - we just verify the handler works
+	_ = body
+}
+
+func TestMetrics_Registry(t *testing.T) {
+	t.Parallel()
+
+	metrics := NewMetrics()
+
+	registry := metrics.Registry()
+	if registry == nil {
+		t.Error("Expected registry, got nil")
+	}
+}