Observability support

Things that we could do:
- Keep counters that track communication statistics between workers, like the number of RPC calls and the amount of data sent/received.
- Expose a hook API so packages can register functions that will be called on sending/receiving data. Not sure about the performance implications of this.
- Read-only API to check the status of a worker, the time of last contact, and state of worker-to-worker connections.

Use cases:
- Tracking network usage between workers on HPC clusters
- Better support for profiling tools like Extrae (https://github.com/JuliaParallel/DistributedNext.jl/issues/1#issuecomment-2437194907)

未指定文件或测试。首先跟踪 DistributedNext.jl 中 worker 之间的通信，并查看链接的 Extrae 讨论；先明确这项贡献涉及计数器、钩子还是 worker 状态 API。完成的标准是确定了双方同意的范围，并且有可观察的通信或性能分析数据。

由索引模型根据 Issue 内容生成。