[Umbrella] Subissue Improve Flink Metrics coverage for Unit and Integration Test

### Search before asking

- [x] I searched in the [issues](https://github.com/alibaba/fluss/issues) and found nothing similar.

### Description

Increasing coverage for the following

```
Warning: Rule violated for class com.alibaba.fluss.flink.metrics.FlinkHistogram.FlinkHistogramStatistics: lines covered ratio is 0.00, but expected minimum is 0.70
Warning: Rule violated for class com.alibaba.fluss.flink.metrics.FlinkCounter: lines covered ratio is 0.00, but expected minimum is 0.70
Warning: Rule violated for class com.alibaba.fluss.flink.metrics.FlinkMeter: lines covered ratio is 0.33, but expected minimum is 0.70
Warning: Rule violated for class com.alibaba.fluss.flink.metrics.FlinkHistogram: lines covered ratio is 0.50, but expected minimum is 0.70
```

### Willingness to contribute

- [x] I'm willing to submit a PR!