prevent string formatting in Entry.Logf when log level is not enabled

entry.go

@@ -298,7 +298,9 @@ func (entry *Entry) Panic(args ...interface{}) {
 // Entry Printf family functions
 
 func (entry *Entry) Logf(level Level, format string, args ...interface{}) {
-	entry.Log(level, fmt.Sprintf(format, args...))
+	if entry.Logger.IsLevelEnabled(level) {
+		entry.Log(level, fmt.Sprintf(format, args...))
+	}
 }
 
 func (entry *Entry) Tracef(format string, args ...interface{}) {
@@ -390,4 +392,4 @@ func (entry *Entry) Panicln(args ...interface{}) {
 func (entry *Entry) sprintlnn(args ...interface{}) string {
 	msg := fmt.Sprintln(args...)
 	return msg[:len(msg)-1]
-}
+}

entry_test.go

@@ -139,3 +139,17 @@ func TestEntryWithIncorrectField(t *testing.T) {
 	assert.Equal(eWithFunc.err, `can not add field "func"`)
 	assert.Equal(eWithFuncPtr.err, `can not add field "funcPtr"`)
 }
+
+func TestEntryLogfLevel(t *testing.T) {
+	logger := New()
+	buffer := &bytes.Buffer{}
+	logger.Out = buffer
+	logger.SetLevel(InfoLevel)
+	entry := NewEntry(logger)
+
+	entry.Logf(DebugLevel, "%s", "debug")
+	assert.NotContains(t, buffer.String(), "debug", )
+
+	entry.Logf(WarnLevel, "%s", "warn")
+	assert.Contains(t, buffer.String(), "warn", )
+}