Too much logs are outputted

When the number of metrics reaches the upper limit, a warn log will be output every time:

```
[ WARN ] [2019-04-11 20:00:42] 65634a0c4570ce860ea9f5410eeedb3a- metrics size > 5000, a nop metric will be returned. name: middleware.tomcat.http.request.path{path=xxx.com/isExistNumber.do}
```

This might introduce the risk to fulfil the disk.
Need to do some trick here. For example, output the warn message a few times and then stop.

No file or test is named. Start by searching for the warning text and the metric-count limit handling, then reproduce repeated over-limit metric creation. Done means the warning output is bounded while the nop metric behavior remains intact.

Written by the indexing model from the issue text.