Thanks to @tomprince for the suggestion.

Copy everything from how the issuer does it.

literalExample is deprecated and overused. See:
https://whetstone.privatestorage.io/privatestorage/PrivateStorageio/-/merge_requests/201#note_16966

Thanks for the suggestion @jcalderone!

Thanks @tomprince!

Response time, probe fails and TLS expiry incl. alerts.

The Blackbox exporter can be used to check whether some services do answer;
We'll use it for our HTTPS endpoints.
Especially handy is it's checking for TLS cert expiry.

This change reflects my intention of having an alert fire when a node
is not scrapable for five minutes or longer.  I hadn't understood the
default avg() function well enough before, yielding an alert that would
fire too soon (i.e. any time a node was down, even only one minute for a
reboot).

Fixes privatestorageops#358.

- The serivice can be reached by other machines.
- The metrics cannot be reached by other machines.

This changes the Throughput graph to be relative, changes its unit
to Bit/s, and fixes the labels of the Network pkt/s graph.

Fixes privatestorageops#24.

Fixes privatestorageops#355.

[Copy pasta](https://whetstone.privatestorage.io/privatestorage/PrivateStorageio/-/blob/ca44204076983ce02fd3e0dacd940514e0821e23/nixos/modules/spending.nix#L130) - Thanks @tomprince!

Epiphany: Systemd reads more than a single environment file. This
lets me improve the file naming much.