Create custom prometheus metrics exporter for NetHSM

The central metrics server for the Arch Linux infrastructure is Prometheus.

To collect metrics of a running NetHSM backend, we require a custom exporter, that can collect the device metrics as well as per-key details (we are particularly interested in the amount of operations). For the former we require a system-wide Metrics user and for the latter we require an untagged Operator user in each namespace, that has access to the key information (but can not in fact use the keys for cryptographic operations).