Add eventexecutor.workers metrics for Netty

[divijvaidya]

Please describe the feature request.
We added a few Netty event executor related metrics at #522. This is a request to add another metrics eventexecutor.workers to the list of Netty metrics. This new metric represents the total number of Netty's event loops.

Rationale
A similar metric was added to Armeria with line/armeria#4675. This is a useful metric while debugging to determine the number of workers without having to explicitly look at the configuration.

Additional context
We could choose to use the reference implementation (Apache v2 license) at https://github.com/line/armeria/blob/024f86dedccc7897a6206770dd95852ca2ed66cc/core/src/main/java/com/linecorp/armeria/common/metric/EventLoopMetrics.java#L88

[shakuzen]

Thanks for opening the issue. It sounds like it's probably generally useful. Could you share more context on when users would use this metric and for what purpose, to help me understand as not a heavy Netty user? Also, would you be willing to submit a pull request for this?