update log/readme.md (#1085)

* update log/readme.md

Co-authored-by: chenzhihui <zhihui_chen@foxmail.com>

log/README.md

@@ -1,27 +1,36 @@
-# Log
+# Logger
 
 ## Usage
 
 ### Structured logging
 
 ```go
-
-var logger log.Logger = log.MultiLogger(log.NewStdLogger(os.Stdout), syslog.NewLogger())
-
+logger := log.NewStdLogger(os.Stdout)
+// fields & valuer
 logger = log.With(logger,
     "service.name", "hellworld",
     "service.version", "v1.0.0",
     "ts", log.DefaultTimestamp,
     "caller", log.DefaultCaller,
 )
-
 logger.Log(log.LevelInfo, "key", "value")
 
-
-var helper log.Helper = log.NewHelper(logger)
+// helper
+helper := log.NewHelper(logger)
 helper.Log(log.LevelInfo, "key", "value")
 helper.Info("info message")
 helper.Infof("info %s", "message")
 helper.Infow("key", "value")
 
+// filter
+log := log.NewHelper(log.NewFilter(logger,
+	log.FilterLevel(LevelInfo),
+	log.FilterKey("foo"),
+	log.FilterValue("bar"),
+	log.FilterFunc(customFilter),
+))
+log.Debug("debug log")
+log.Info("info log")
+log.Warn("warn log")
+log.Error("warn log")
 ```