Logging: Add journald support

[dumbbell]

The implementation depends on erlang-systemd which uses Unix socket support introduced in Erlang 19. Therefore it doesn't rely on a native library. We also don't need special handling if the host doesn't use journald.

To enable the journald handler, add the following configuration variable:

log.journald = true

The log level can also be set the same way it is with other handlers:

log.journald.level = debug

The log messages are communicated to journald using structured data. It is possible to configure which fields are transmitted and how they are named:

log.journald.fields = SYSLOG_IDENTIFIER="rabbitmq-server" syslog_timestamp syslog_pid priority ERL_PID=pid

In this example:

• the SYSLOG_IDENTIFIER is set to a string literal
• syslog_timestamp and syslog_pid` are aliases for SYSLOG_TIMESTAMP=time and SYSLOG_PID=os_pid
• priority is a special field computed from the log level
• ERL_PID=pid indicates pid should be sent as the ERL_PID field.

The message itself is implicit and always sent. Otherwise, the list of fields must be exhaustive: fields which are unset in a particular log event meta are sent as an empty string and non-mentionned fields are not sent. The order is not important.

Here are some messages printed by journalctl -f during RabbitMQ startup:

Mar 26 11:58:31 ip-172-31-43-179 rabbitmq-server[19286]: Ready to start client connection listeners
Mar 26 11:58:31 ip-172-31-43-179 rabbitmq-server[19286]: started TCP listener on [::]:5672
Mar 26 11:58:31 ip-172-31-43-179 rabbitmq-server[19286]: Server startup complete; 0 plugins started.

[dumbbell]

Note that erlang-systemd is only used for its journald support in this pull request. I plan to change how we tell systemd RabbitMQ is ready to use this new dependency as well. This will allow us to get rid of the previous methods (socat and sd_notify). This will come in a separate patch.

[dumbbell]

Here is how I tested the patch:

1. I used a Debian Buster VM because journald requires systemd as far as I know.

2. To view the logs, I just used the "tail -f" journald equivalent:

   journalctl -f
   

3. I configured RabbitMQ to use both journald and stdout:

   cat >/tmp/rabbitmq.conf <<EOF
   log.console = true
   log.journald = true
   EOF
   

4. I started RabbitMQ with that configuration file:

   gmake run-broker RABBITMQ_CONFIG_FILES=/tmp/rabbitmq.conf