Since ZoL frees ARC under memory pressure, let's not
count it as "used" but instead as "free" memory.

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

Or rather, let it listen only on localhost.
I thought Grafana needed it, or Promtail needed it, but I don't remember
clearly, the web doesn't say clearly, and on my local dev stack
Promtail/Loki seems to still work just fine without GRPC on the network.

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 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

Also strip that domain component from the labels collected.

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

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.