Add hours query param to `getDataUsage` endpoint

The usage is currently returns the usage of last 30 days. This PR adds a query to specify the last number of hours. ``` GET /metrics/transfer # Default: 30 days (720 hours) GET /metrics/transfer?last=24h # Last 24 hours GET /metrics/transfer?last=1w # Last 7 days GET /metrics/transfer?last=30d # Last 30 days (explicit) ```