feat: add flow_statistics system table and SHOW FLOW STATUS for flow runtime observability

Summary

It would be useful to expose flow runtime status through both a system table and a SQL command, so users can quickly inspect flow health and basic runtime statistics.

Proposed Feature

Add:

• a system table such as flow_statistics
• a SQL command such as SHOW FLOW STATUS

to expose information like:

• flow start time
• uptime / running duration
• last 3 errors
• total amount of data processed since startup
• if feasible, basic resource usage such as memory and CPU

Motivation

Today it is hard to quickly understand whether a flow is healthy and how it has behaved since startup. A lightweight built-in status view would make debugging and daily operations much easier.

This could be a good first issue if the initial implementation is scoped to core runtime metadata first, with resource metrics included when feasible.

Checklist

• Add a system table (for example flow_statistics) that exposes per-flow runtime statistics such as start time, uptime, processed data volume, and recent errors.
• Add a SHOW FLOW STATUS SQL command that provides a user-friendly way to inspect the same runtime status, with optional CPU/memory metrics when available.