add ByteBuf monitor

[342963711]

Under the default configuration, the direct memory used by netty cannot be observed through MBeans. Hope to add ByteBuf's monitor

[franz1981]

If you set -Dio.netty.maxDirectMemory=0 or disable Unsafe, ByteBufs should use Cleaners and should be visible as any other direct memory user. Without, you need to use the allocator statistics or, if you're using an higher-level framework using Netty, it is often possible to read such metrics with the right micrometer plugin.