add cute architecture graph from upstream with more explanations

although those are limited enough that we use a separate graphing

layer on top (see [[Grafana]]).

Basic design

------------

The Prometheus web interface is available at:

<https://prometheus.torproject.org>

`Execute`. For example, [this link](https://prometheus1.torproject.org/graph?g0.range_input=2w&g0.expr=node_load5&g0.tab=0) will show the 5-minute load

over the last two weeks for the known servers.

All machines configured through Puppet are scraped by the central

server every 15 seconds.

Here you can see, from the [Prometheus overview documentation](https://prometheus.io/docs/introduction/overview/) the

basic architecture of a Prometheus site:

<img src="https://prometheus.io/assets/architecture.png" alt="A

drawing of Prometheus' architecture, showing the push gateway and

exporters adding metrics, service discovery through file_sd and

Kubernetes, alerts pushed to the Alertmanager and the various UIs

pulling from Prometheus" />

As you can see, Prometheus is somewhat tailored towards

[Kubernetes](https://kubernetes.io/) but it can be used without it. We're deploying it with

the `file_sd` discovery mechanism, where Puppet collects all exporters

into the central server, which then scrapes those exporters every

`scrape_interval` (by default 15 seconds). The architecture graph also

shows the Alertmanager which could be used to (eventually) replace our

Nagios deployment.

It does not show that Prometheus can federate to multiple instances

and the Alertmanager can be configured with High availability.

Munin expatriates

-----------------