Add 'tarsnap' plugin.

Monitors the amount of data stored by the local machine on the tarsnap.com service.

plugins/tarsnap/README.md

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+Munin plugin for Tarsnap
+========================
+
+https://github.com/warrenguy/munin-tarsnap
+
+This plugin creates two graphs:
+
+* *tarsnap_total* - summarising the total amount of data the local tarsnap
+instance has stored on the service (total and compressed).
+* *tarsnap_unique* - summarising the total amount of unique (deduplicated)
+data the local tarsnap instance has stored on the service (total and 
+compressed). The compressed value here is the actual amount of data stored
+on the tarnap servers and what tarsnap uses for billing.
+
+Usage
+-----
+
+Add the following to your backup script (after tarsnap has run), or to a 
+cron job:
+
+    /usr/local/bin/tarsnap --print-stats > /path/to/tarsnap-stats.txt
+ 
+N.B.: ensure `/path/to/munin-stats.txt` is readable by munin-node.
+
+Configuration
+-------------
+
+Define the path to the stats file created above in your munin-node
+configuration:
+
+    [tarsnap]
+    env.STATSFILE /path/to/tarsnap-stats.txt
+
+The default value is `/var/lib/munin/tarsnap-stats.txt`.
+
+Author
+------
+
+Warren Guy <warren@guy.net.au>
+
+https://warrenguy.me
+
+Copyright
+---------
+
+Copyright (C) 2014 Warren Guy <warren@guy.net.au>