[KYUUBI #4546] Fully exclude metrics dir from git

### _Why are the changes needed?_

Follow our attitude towards other excluded folders, exclude the entire `metrics` folder.
Like in `.gitignore`
```
embedded_zookeeper/
```

`metrics` dir will be created in source code when run `KyuubiServer` in local IDE.

Close #4546

### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible

- [ ] Add screenshots for manual tests if appropriate

- [ ] [Run test](https://kyuubi.readthedocs.io/en/master/develop_tools/testing.html#running-tests) locally before make a pull request

Closes #4547 from zwangsheng/KYUUBI_4546.

Closes #4546

8f13670e6 [zwangsheng] [KYUUBI #4546] Ignore full metrics dir in git version control

Authored-by: zwangsheng <2213335496@qq.com>
Signed-off-by: Cheng Pan <chengpan@apache.org>

.gitignore

@@ -58,8 +58,7 @@ metastore_db
 derby.log
 rest-audit.log
 **/dependency-reduced-pom.xml
-metrics/report.json
-metrics/.report.json.crc
+metrics/
 /kyuubi-ha/embedded_zookeeper/
 embedded_zookeeper/
 /externals/kyuubi-spark-sql-engine/operation_logs/