`kdaectrl`: metrics

It would be good to have metrics for things like:
- hardware readback errors
- setpoint errors (be it kafka or to the hardware)
- current queue length
- active threads
- last values for ca monitors

and several others

This ticket is to add a framework to serve metrics and add the above and any other appropriate metrics.

No file or test is named. Start by locating the kdaectrl entry point and inventorying how hardware readbacks, setpoints, CA monitors, queues, and threads are handled. Done means a metrics-serving framework exists and exposes the listed metrics plus any agreed additions.

Vom Indexierungsmodell aus dem Issue-Text verfasst.