Alerting: Log when alert rule cannot be screenshot to help debugging (#58537) (#58563)

(cherry picked from commit 68600c224b)

pkg/services/ngalert/image/service.go

@@ -124,11 +124,15 @@ func NewScreenshotImageServiceFromCfg(cfg *setting.Cfg, db *store.DBstore, ds da
 // alert rule has a Dashboard UID and the dashboard exists, but does not have a
 // Panel ID in its annotations then an ErrNoPanel error is returned.
 func (s *ScreenshotImageService) NewImage(ctx context.Context, r *models.AlertRule) (*models.Image, error) {
+	logger := s.logger.FromContext(ctx)
+
 	if r.DashboardUID == nil || *r.DashboardUID == "" {
+		logger.Debug("Cannot take screenshot for alert rule as it is not associated with a dashboard")
 		return nil, ErrNoDashboard
 	}
 
 	if r.PanelID == nil || *r.PanelID == 0 {
+		logger.Debug("Cannot take screenshot for alert rule as it is not associated with a panel")
 		return nil, ErrNoPanel
 	}
 
@@ -138,14 +142,12 @@ func (s *ScreenshotImageService) NewImage(ctx context.Context, r *models.AlertRu
 		Timeout:      screenshotTimeout,
 	}
 
+	logger = logger.New("dashboard", opts.DashboardUID, "panel", opts.PanelID)
+
 	// To prevent concurrent screenshots of the same dashboard panel we use singleflight,
 	// deduplicated on a base64 hash of the screenshot options.
 	optsHash := base64.StdEncoding.EncodeToString(opts.Hash())
 
-	logger := s.logger.FromContext(ctx).New(
-		"dashboard", opts.DashboardUID,
-		"panel", opts.PanelID)
-
 	// If there is an image is in the cache return it instead of taking another screenshot
 	if image, ok := s.cache.Get(ctx, optsHash); ok {
 		logger.Debug("Found cached image", "token", image.Token)