Besides migrating all charts to the faster and more capable Grafana 8
TimeSeries charting tool this mainly introduces two panels to view
the logs of PaymentServer.  It has some naive and minimal filtering
in place to only show lines that are not caused by the metrics gathering
itself or that I just deemed to be OK (like "GET" requests) so we
hopefully catch those dreaded "Unexpected Exception" lines.

Fixes privatestorageops#207.

Should be pure refactoring

This is my latest version of this, updated to work with the
packages in NixOS 21.05.

Thanks to @jcalderone for the suggestion!

Fixes privatestorageops#408

Take what I learned from @tomprince in
https://whetstone.privatestorage.io/privatestorage/PrivateStorageio/-/merge_requests/220#note_17605 :
Assigning a service a `startAt' will make timer.target start it,
no need to also have it `wantedBy' multiuser.target.

Also add megacli2prom to the path even if it should be already because
of megacli2prom.  Better be safe than sorry.

Thanks go out to @tomprince!

Tryfix the starting of the timer before tahoe is updated.

This adds a detailed dashboard for tahoe-lafs running on one node.

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!

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.

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.