Health-Metrics for sftp-server

[KilianDargel]

In production, 2 problems regarding the health-status of the sftp-server go unnoticed.
The sftp-server suddenly stops receiving files regularly, or they are not picked up anymore regularly.
We discussed in #9808 if a mechanism is needed to track if timeouts are happening on either side.

Imo, incoming senders should monitor their own file-pushing, i.e. add the sftp-server as a pingexporter-target. (CGW/rpg already allow this option).
But would a check if files are piling up on the server make sense, or should that be monitored on the outgoing containers as well (ipsec/BGP on CGW)?