Add FormatPrepare to ConsoleWriter.

console.go

@@ -76,6 +76,8 @@ type ConsoleWriter struct {
 	FormatErrFieldValue Formatter
 
 	FormatExtra func(map[string]interface{}, *bytes.Buffer) error
+
+	FormatPrepare func(map[string]interface{}) error
 }
 
 // NewConsoleWriter creates and initializes a new ConsoleWriter.
@@ -124,6 +126,13 @@ func (w ConsoleWriter) Write(p []byte) (n int, err error) {
 		return n, fmt.Errorf("cannot decode event: %s", err)
 	}
 
+	if w.FormatPrepare != nil {
+		err = w.FormatPrepare(evt)
+		if err != nil {
+			return n, err
+		}
+	}
+
 	for _, p := range w.PartsOrder {
 		w.writePart(buf, evt, p)
 	}

console_test.go

@@ -418,6 +418,29 @@ func TestConsoleWriterConfiguration(t *testing.T) {
 		}
 	})
 
+	t.Run("Sets FormatPrepare", func(t *testing.T) {
+		buf := &bytes.Buffer{}
+		w := zerolog.ConsoleWriter{
+			Out: buf, NoColor: true, PartsOrder: []string{"level", "message"},
+			FormatPrepare: func(evt map[string]interface{}) error {
+				evt["message"] = fmt.Sprintf("msg=%s", evt["message"])
+				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 msg=Foobar\n"
+		actualOutput := buf.String()
+		if actualOutput != expectedOutput {
+			t.Errorf("Unexpected output %q, want: %q", actualOutput, expectedOutput)
+		}
+	})
+
 	t.Run("Uses local time for console writer without time zone", func(t *testing.T) {
 		// Regression test for issue #483 (check there for more details)