Calculate depth per-handler class in netty channel pipeline instrumentation

[labbati]

Our previous implementation of the netty's channel pipeline assumed that the only way to add ChannelHandler was through ChannelPipeline#add* methods called from some other service.

In reality it might happen (and this is what happens with reactor 0.8.* when it registers the HttpServerCodec that the addLast method is called from a concrete channel handler extending a ChannelInitializer. In this case new handler can be added while the parent channel handler is added, where the the initChannel method can be used to add additional handlers.

This PR adds the possibility to handle such cases, while preventing recursions that our pre-existing safety nets (that caused the new version of reactor to not work) where there to prevent