fix(metrics): fallback to mock writer on file open failure

When timestamps are enabled but the target file cannot be opened,
NewZerologMetrics returned nil. This caused a nil pointer dereference
on subsequent metrics.Capture() calls.

This fix gracefully degrades to a no-op mockWriter and logs a warning.
Adds a regression test to verify the fallback behavior.

Fixes #595

Signed-off-by: abhaygoudannavar <abhaysgoudnvr@gmail.com>

Author: abhaygoudannavar

internal/metrics/metrics.go

@@ -18,6 +18,7 @@ import (
 	"os"
 
 	"github.com/rs/zerolog"
+	"github.com/sirupsen/logrus"
 )
 
 type Writer interface {
@@ -57,7 +58,8 @@ func NewZerologMetrics(enabled bool, target string, containerID string) Writer {
 		zerolog.TimeFieldFormat = zerolog.TimeFormatUnixNano
 		file, err := os.OpenFile(target, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0666)
 		if err != nil {
-			return nil
+			logrus.Warnf("Failed to open metrics file %s: %v. Metrics will be disabled.", target, err)
+			return &mockWriter{}
 		}
 		logger := zerolog.New(file).Level(zerolog.InfoLevel).With().Timestamp().Logger()
 		return &zerologMetrics{

internal/metrics/metrics_test.go

@@ -52,3 +52,15 @@ func TestZerologMetricsMetadata(t *testing.T) {
 		t.Errorf("timestampOrder = %v, want 0", got)
 	}
 }
+
+func TestZerologMetricsInvalidFileDoesNotPanic(t *testing.T) {
+	// Provide a path to a directory that definitely does not exist
+	invalidPath := "/does/not/exist/timestamps.log"
+	
+	// Create the metrics writer with timestamps enabled but an invalid path
+	writer := NewZerologMetrics(true, invalidPath, "container-test")
+	
+	// This call would panic with a nil pointer dereference without the fix.
+	// With the fix, it will safely execute the mockWriter's no-op Capture().
+	writer.Capture(TS00)
+}