Since the change to async logger, this test fails 100% of the time for me locally. Steve Rowe confirmed that it fails for him too so it's not something weird in my environment.
Why there are no reports from Jenkins of this failure is a mystery.
The test succeeds, but there is a thread leak and the stack trace points to "lmax.disruptor" which is certainly a part of the async logging.
And why, if this is something generic, it doesn't fail many many tests is another mystery so I suspect it's something specific to the test, perhaps having to do with mocking the metric reporter, but that's a wild shot in the dark.
My working hypothesis is that "something isn't being closed/shutdown correctly", but that's...a little vague.
Sep 04, 2018 9:38:00 AM com.carrotsearch.randomizedtesting.ThreadLeakControl checkThreadLeaks
WARNING: Will linger awaiting termination of 1 leaked thread(s).
Sep 04, 2018 9:38:20 AM com.carrotsearch.randomizedtesting.ThreadLeakControl checkThreadLeaks
SEVERE: 1 thread leaked from SUITE scope at org.apache.solr.metrics.SolrMetricReporterTest:
1) Thread[id=14, name=Log4j2-TF-1-AsyncLoggerConfig--1, state=TIMED_WAITING, group=TGRP-SolrMetricReporterTest]
at sun.misc.Unsafe.park(Native Method)