[bug] Timeseries migration script does not handle singal metrics

[pandafy]

In PR #368, we modified the structure for storing metrics in the time series database. However, this update did not include restructuring the signal metrics. As a result, the signal metrics are still being written with interface name as the metric key.

Additionally, the time series migration script introduced in PR #368 does not account for signal metrics and would inadvertently delete signal data if executed on the database.

Required fixes

1. Restructure Signal Metrics: Update the signal metrics to align with the traffic and wifi_clients metrics.
2. Update Migration Script: Modify the time series migration script to handle signal metrics correctly and prevent data loss.