issue-405: quick fix for accidentally removed instrumentations

[mitchnull]

Quick fix for accidentally removed instrumentations in ExecutorServiceMetricsBinder:

• just return the original executorService instead of the unwrapped io.netty class

The proper fix would be to rethink the original fix for #62 and find a less brittle solution, but in the meantime this obvious error should be fixed IMO.

It would also be nice if it were backported to previous stable branches

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[sdelamo]

@jeremyg484 is this necessary?

[jeremyg484]

@sdelamo It seems OK to me on the surface, but I haven't tested it thoroughly enough to be confident it won't break other things downstream (as Graeme mentioned in his comment on #405). I took his comment there to be an ok workaround, but maybe I'm misunderstanding.

[jeremyg484]

I have spent some further time digging into this in relation to #679 and it's PR #702. I now have a better understanding of the intended behavior, and I think it is safe to close this PR without merging. I've confirmed that a Netty EventLoopGroup getting wrapped byInstrumentedExecutorService will still cause a class cast exception in NettyHttpServer, thus we should continue to return the unwrapped instance as a precaution - that said I haven't found any Micronaut code that does such wrapping on EventLoopGroup now.