[KYUUBI #4546][FOLLOWUP] only exclude `metrics` dir in root directory of repo

[Z1Wu]

Why are the changes needed?

Follow-Up of patch: https://github.com/apache/kyuubi/pull/4547

The modification introduced by https://github.com/apache/kyuubi/pull/4547 will result in all folder whose name is metrics being ignored.

# for example, a normal metrics package  for spark-engine will be ignored
externals/kyuubi-spark-sql-engine/src/main/scala/org/apache/spark/kyuubi/metrics/

This patch changes the rule in .gitignore to only exclude metrics directory created by JSON reporter in root directory of REPO which is usually working directory of debug process.

Close https://github.com/apache/kyuubi/issues/4546

Simple test

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 locally before make a pull request

[Z1Wu]

cc @zwangsheng @pan3793

[pan3793]

Has anyone tested this scenario?

https://github.com/apache/kyuubi/issues/4546#issuecomment-1905252699