This has triggered an OOM in a process which was churning through s3a fs
instances; the increased memory footprint of IOStatistics amplified what
must have been a long-standing issue with FS instances being created
and not closed()
* Makes sure instrumentation is closed when the FS is closed.
* Uses a weak reference from metrics to instrumentation, so even
if the FS wasn't closed (see HADOOP-18478), this back reference
would not cause the S3AInstrumentation reference to be retained.
* If S3AFileSystem is configured to log at TRACE it will log the
calling stack of initialize(), so help identify where the
instance is being created. This should help track down
the cause of instance leakage.
Contributed by Steve Loughran.
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