Github user zentol commented on the issue:
https://github.com/apache/flink/pull/2517
that wouldn't solve the concurrency issues. If you have 2 concurrent
notifies going on you would pass the same FrontMetricGroup to the reporter,
hence for both calls the reporter would work on the same referenced
MetricGroup, even though they may be supposed to be different ones.
I would propose keeping the current approach of creating a new
FrontMetricGroup every time. This means you can simplify that class a lot,
since you can now extend `ProxyMetricGroup`, and pass the index through the
constructor as well. I don't see a solution where we can get away without
creating a new object or putting the entire notification inside a synchronized
block (which would be a no-go).
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