Add metrics for the Netty internals

Firstly, there is nothing in the docs regarding what metrics are collected, and how to disable metrics collections. I asked in https://github.com/yidongnan/grpc-spring-boot-starter/issues/239 if more details could be added.

Then, looking at the metrics collecting interceptors, it seems the request and response are counted globally without any reference to the RPC method. This doesn't really make sense, because either the client or the server could send streaming messages that'd increment these counter. This is misleading since the timer times the complete call, not individual request/response.

Lastly, thread pool information on the server and client side is crucial to have.

I've a project that addresses the last two issues; it's not based on Spring Boot (deliberate choice), or grpc-spring-boot-starter, but uses Micrometer.

https://github.com/asarkar/kotlin/tree/master/grpc-learning