Add 2 very useful endpoints

[WenzzyX]

Hello. I ran into the problem of normal counting of the transferred data, if there are many keys, then statistics are obtained for all keys, therefore, I ask you to add 2 endpoints.

1. {SECRET_URL}/:id/data-transferred - Returns transferred data for one key by id
2. {SECRET_URL}/data-transferred-keys (Request body : { keys: [1, 2, 3 ... n] }) - Returns the passed data for all keys that are specified in the request (IDs)

[skala-vl]

A very necessary function! Please implement!

[Blueclaus13]

Could you assign me this issue?

[maddyhof]

@Blueclaus13 sorry for the delay over here – would you still like to work on this? We'd love that!

[fortuna]

@WenzzyX you can access the metrics directly from the Prometheus metrics backend: https://stackoverflow.com/questions/76869983/monitoring-outline-vpn-how-to-access-performance-metrics-of-an-outline-server