Monitoring: Add dashboard and alerting for used swap

Closes privatestorageops#358

See merge request !198

Alerting: Don't when no scraping error ever happened.

See merge request !197

Use a version of vulnix that doesn't collapse derivations with different sets of patches.

See merge request !196

Update PaymentServer.

See merge request !195

Expose zkap-spending-service metrics.

See merge request !184

Fixes privatestorageops#358.

Monitoring: Add dash for scraping failures + alert

Closes privatestorageops#355

See merge request !188

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

Don't use our nixpkgs fork for LeaseReport.

Closes privatestorageops#373

See merge request !192

Update nixpkgs.

See merge request !193

Protocol sequence diagrams for ZKAPs

See merge request !190

now that default.nix is actually somewhat useful

`cp --recursive ...` is required for permission issues
`... --out-link result-docs` is required for path collision issues
the rest is at my discretion

Monitoring dashboards: Rethink scales

Closes privatestorageops#24

See merge request !189

Add a nixos module for the ZKAP Spending Service

See merge request !172

Simplify local grid hardware

See merge request !182

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.

Metric names: It's vouchers (plural)

See merge request !187

Make payment server answer with a 404 by default

See merge request !186

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

Enable alerting

Closes privatestorageops#392

See merge request !185

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