This is a simple exporter for various statistics relating to a Postfix
server.


# Features

* Examines mail queue periodically and exports `postfix_mail_queue_size`;

* Reads syslog entries as they happen, and exports disposition status
  counters (`postfix_delivery_delays_count`) per DSN, as well as delay summaries per
  DSN (`postfix_delivery_delays{quantile="..."}`);

* Total number of SMTP connections (`postfix_smtpd_connections_total`) and
  currently-active connections (`postfix_smtpd_active_connections`);

* Count how many delivery attempts were received
  (`postfix_incoming_delivery_attempts_total`), split out by whether we
  accepted or rejected the message (`status`) and the exact DSN provided to
  the client (`dsn`);

* Whether or not the Postfix `master` process is running (`postfix_up`);

* Drinks from the syslog stream directly.


# Deployment

The default way of rolling out this exporter is as a Docker container.  For
that, you'll want to setup a volume for the postfix queue (making it
available in the container as `/var/spool/postfix`), and some env
vars.  You can also run it directly (via the gem), with the same env vars,
and with the expectation that `/var/spool/postfix` is in the usual place.

If you configure the postfix-exporter to run in the same PID namespace as
whatever it is that's running Postfix itself (either `--pid=host` or in the
same NS namespace, a la k8s pods), then the `postfix_up` metric will be
valid, otherwise it'll be random (but unlikely to be correct).


## Environment Variables

* `SYSLOG_SOCKET` -- where to read raw syslog events from.  If you're
  running Postfix in an isolated container, you can probably hook up
  `/dev/log` directly between containers with a bit of shenanigans,
  otherwise configure your syslog daemon to write out a copy of all `mail`
  facility messages to a custom socket.  Alternately, if you use logstash,
  take a look at the [`syslogstash`
  gem](https://rubygems.org/gems/syslogstash) and
  [container](https://hub.docker.com/r/discourse/syslogstash/), and go forth
  and sin no more.