node_memory_MemAvailable_bytes is a better estimator than the sum I used before
says some Prometheus documentation.  It is also almost the same, but reads nicer.

This should fix the current alerts for our RAID arrays.
It's only "should" because I can't test it proper without said
RAID arrays in the dev or staging machines.

The newer "Time Series" panel does not support two axes.

- Also alert on "negative" (== receiving) errors
- Use 'rate' so we can get out of the reporting if situation normalizes

Grafana 8 recommends these for added performance and capabilities.

- (tryfix) Switch to Loki datasource;
- Filter requests to metrics instead of all GET;
- Show the last week by default
- Show log times

Interesting and not quickly fixable as it looks like: the
GET ... line comes *after*  the result.  We might be logging
this wrong to begin with, but probably this is some next
gen AI line ordering intelligence

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.

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

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

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.

Only dashboards in this dir