Update documentation

README.md

@@ -2,6 +2,8 @@
 
 Copyright (c) 2015-2023 Paul D. Gear <https://libertysys.com.au/>
 
+Juju layer copyright (c) 2017-2018 Canonical Ltd <https://charmhub.io/ntp>
+
 ## License
 
 GPLv3 - see COPYING.txt for details
@@ -12,8 +14,8 @@ GPLv3 - see COPYING.txt for details
 NTPmon is a program which is designed to report on essential health metrics for
 NTP.  It provides a Nagios check which can be used with many alerting systems,
 including support for Nagios performance data.  NTPmon can also run as a daemon
-for sending metrics to collectd or telegraf.  It supports both `ntpd` and
-`chronyd`.
+for sending metrics to collectd, prometheus, or telegraf.  It supports both
+`ntpd` and `chronyd`.
 
 
 ## Prerequisites
@@ -22,12 +24,15 @@ NTPmon is written in python, and requires python 3.3 or later.  It uses modules
 from the standard python library, and also requires the `psutil` library, which
 is available from pypi or your operating system repositories. NTPmon also
 requires `ntpq` and `ntptrace` from the NTP distribution (or `chronyc` if you're
-using chrony instead).
+using chrony).
 
 On Ubuntu (and probably other Debian-based Linux distributions), you can install
 all the prerequisites by running:
 
-    sudo apt-get install ntp python3-psutil
+    sudo apt-get install ntp python3-prometheus-client python3-psutil
+
+(The python3-prometheus-client package is only needed if you intend to run in
+prometheus exporter mode - see below.)
 
 ## Usage
 
@@ -38,7 +43,6 @@ To start running NTPmon, run:
     cd ntpmon
     ./src/ntpmon.py --help
 
-
 ## Metrics
 
 NTPmon alerts on the following metrics of the local NTP server:
@@ -62,19 +66,18 @@ than 75% total reachability of all configured peers.
 
 #### offset
 
-Is the clock offset from its sync peer (or other peers, if the sync peer
-is not available) acceptable?  Return CRITICAL for 50 milliseconds or more
-average difference, WARNING for 10 ms or more average difference, and OK
-for anything less.
+Is the clock offset from its sync peer (or other peers, if the sync peer is not
+available) acceptable?  Return CRITICAL for 50 milliseconds or more average
+difference, WARNING for 10 ms or more average difference, and OK for anything
+less.
 
 #### traceloop
 
-Is there a sync loop between the local server and the stratum 1 servers?
-If so, return CRITICAL.  Most public NTP servers do not support tracing,
-so for anything other than a loop (including a timeout), return OK.
-Traceloop is disabled by default and may be deprecated in a future
-release, since it produces additional NTP traffic which is not useful in
-most cases.
+Is there a sync loop between the local server and the stratum 1 servers? If so,
+return CRITICAL; for anything other than a loop (including a timeout), return
+OK.  Most public NTP servers do not support tracing, so using this produces
+additional NTP traffic which is not useful in most cases. Trace loop detection
+is deprecated, disabled by default, and is not supported for prometheus.
 
 ### System metrics
 
@@ -91,6 +94,15 @@ server (using `ntpq -nc readvar`):
 See the [NTP documentation](http://doc.ntp.org/current-stable/ntpq.html#system)
 for the meaning of these metrics.
 
+### Prometheus exporter
+
+When run in prometheus mode, NTPmon uses the [prometheus python
+client](https://pypi.python.org/pypi/prometheus_client) to expose metrics via
+the HTTP server built into that library.  No security testing or validation has
+been performed on this library by the NTPmon author; users are suggested not to
+expose it on untrusted networks, and are reminded that - as stated in the GNU
+General Public License terms - this software comes with no warranty.
+
 ## Changes from previous version
 
 NTPmon has been rewritten from version 1.0.0 of check_ntpmon.  Changes from
@@ -112,15 +124,13 @@ the original check_ntpmon are:
 - Removed support for changing thresholds; if the one person on the Internet
   who actually uses this really wants it, I might add it back. :-)
 
-
 ## Startup delay
 
 By default, until ntpd has been running for 512 seconds (the minimum time for
 8 polls at 64-second intervals), check_ntpmon will return OK (zero return code).
 This is to prevent false positives on startup or for short-lived VMs.  To
 ignore this safety precaution, use --run-time with a low number (e.g. 1 sec).
 
-
 ## To do
 
 - Better/more documentation.