message: docs/usage: expand logging page
  This clarifies a few existing details related to log levels and
  introduces a section covering `journalctl` and how to inspect logs
  on a live system.
diff: @@ -5,16 +5,37 @@ parent: Usage
  # Logging
  -Zincati supports logging at multiple levels (trace, debug, info, warning, error). Usually only log messages at or above `info` level are emitted.
  +Zincati supports logging at multiple levels (trace, debug, info, warning, error). Usually only log messages at or above warning level are emitted.
  Log verbosity can be increased by passing multiple `-v` flags as command-line arguments.
  -## Tweaking service verbosity
  +## Tweaking agent verbosity
  -Logging verbosity for Zincati service can be tweaked via systemd drop-in files.
  +By default, the Zincati agent is started with info level logging enabled (i.e. `-v`). However, logging verbosity can be freely tweaked via systemd drop-in files.
  -For example, debug logging (`-vv`) can be enabled by creating a drop-in file `/etc/systemd/system/zincati.service.d/10-verbosity.conf` with the following contents:
  +For example, debug logging (`-vv`) can be enabled by creating a drop-in file at `/etc/systemd/system/zincati.service.d/10-verbosity.conf` with the following contents:
  ```
  [Service]
  Environment=ZINCATI_VERBOSITY="-vv"
  ```
  +
  +The maximum level (`-vvv`) goes up to trace-log messages and can be very verbose. It is only meant for development/debugging and for short timespans.
  +It is recommended to not max out log level in production and for long periods of time, as it reduces the signal-to-noise ratio and can easily saturate further log-persisting systems.
  +
  +## Inspecting logs
  +
  +By default Zincati runs as a systemd service, and its log messages are captured by systemd-journald.
  +
  +Most recent logs can be inspected via `sudo journalctl -b 0 -e -u zincati.service`. The resulting output may look like this:
  +
  +```
  +-- Logs begin at Sat 2020-09-12 16:12:13 UTC, end at Wed 2020-09-30 12:52:05 UTC. --
  +Sep 23 10:48:27 localhost systemd[1]: Started Zincati Update Agent.
  +Sep 23 10:48:27 localhost zincati[678]: [INFO ] starting update agent (zincati 0.0.12)
  +Sep 23 10:48:34 localhost zincati[678]: [INFO ] Cincinnati service: https://updates.coreos.fedoraproject.org
  +Sep 23 10:48:34 localhost zincati[678]: [INFO ] agent running on node '<ID>', in update group '<GROUP>'
  +Sep 23 10:48:34 localhost zincati[678]: [INFO ] initialization complete, auto-updates logic enabled
  +...
  +```
  +
  +Optionally, `journalctl` allows to follow log messages emitted in real time by additionally passing a `-f` flag.
  