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.

Use `$__file{}` for grafana's slack URL secret.

See merge request !191

Weekly nixpkgs update.

See merge request !199

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