- Add TimeSeriesStorage with InfluxDB client, write API, and query API for org/bucket
- Add metric types: MetricPoint, UsageMetrics, SystemMetrics, ChargingMetrics with subscriber/IMSI/transaction fields
- Add StoreUsageMetrics, StoreSystemMetrics, StoreChargingMetrics, and StorePacketMetrics to write metrics to InfluxDB
- Add GetUsageStats to query subscriber usage statistics for time range with last aggreg