ExecutorServiceParallelExecutorTest#ensureMetricsThreadDoesntLeak in PR #4965 does not pass in gradle

This is a new test being added to ensure threads don't leak. The failure seems to indicate that threads do leak.

This test fails using gradle but previously passed using maven

PR: https://github.com/apache/beam/pull/4965

Presubmit Failure: 
* https://builds.apache.org/job/beam_PreCommit_Java_GradleBuild/4059/
* https://scans.gradle.com/s/grha56432j3t2/tests/jqhvlvf72f7pg-ipde5etqqejoa?openStackTraces=WzBd

Imported from Jira [BEAM-4088](https://issues.apache.org/jira/browse/BEAM-4088). Original Jira may contain additional context.
Reported by: echauchot.