Github user bowenli86 commented on the issue:
https://github.com/apache/flink/pull/5501
LGTM generally.
I still feel having all the `instanceof` in `notifyOfAddedMetric` and
`notifyOfRemovedMetric` is a bit inelegant. I'm fine with it since there'll
(hopefully) be only a limited number of metric types, so the `instanceof`
clauses won't grow insanely.
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