Support extra arbitrary data at the end of console log (#416)

console.go

@@ -74,6 +74,8 @@ type ConsoleWriter struct {
 	FormatFieldValue    Formatter
 	FormatErrFieldName  Formatter
 	FormatErrFieldValue Formatter
+
+	FormatExtra func(map[string]interface{}, *bytes.Buffer) error
 }
 
 // NewConsoleWriter creates and initializes a new ConsoleWriter.
@@ -128,10 +130,18 @@ func (w ConsoleWriter) Write(p []byte) (n int, err error) {
 
 	w.writeFields(evt, buf)
 
+	if w.FormatExtra != nil {
+		err = w.FormatExtra(evt, buf)
+		if err != nil {
+			return n, err
+		}
+	}
+
 	err = buf.WriteByte('\n')
 	if err != nil {
 		return n, err
 	}
+
 	_, err = buf.WriteTo(w.Out)
 	return len(p), err
 }

console_test.go

@@ -375,6 +375,29 @@ func TestConsoleWriterConfiguration(t *testing.T) {
 			t.Errorf("Unexpected output %q, want: %q", actualOutput, expectedOutput)
 		}
 	})
+
+	t.Run("Sets FormatExtra", func(t *testing.T) {
+		buf := &bytes.Buffer{}
+		w := zerolog.ConsoleWriter{
+			Out: buf, NoColor: true, PartsOrder: []string{"level", "message"},
+			FormatExtra: func(evt map[string]interface{}, buf *bytes.Buffer) error {
+				buf.WriteString("\nAdditional stacktrace")
+				return nil
+			},
+		}
+
+		evt := `{"level": "info", "message": "Foobar"}`
+		_, err := w.Write([]byte(evt))
+		if err != nil {
+			t.Errorf("Unexpected error when writing output: %s", err)
+		}
+
+		expectedOutput := "INF Foobar\nAdditional stacktrace\n"
+		actualOutput := buf.String()
+		if actualOutput != expectedOutput {
+			t.Errorf("Unexpected output %q, want: %q", actualOutput, expectedOutput)
+		}
+	})
 }
 
 func BenchmarkConsoleWriter(b *testing.B) {