improve munin stats (stats_gatherer)

[cl]

Hi,

I have been asked by Zooko to open this ticket, so here it goes:

The statistics gathered by the stats_gatherer can be nicely graphed using munin (file misc/operations_helpers/munin/tahoe_stats). While that file is not terribly difficult to modify/extend/adjust, the default one is missing a few rather important fields (storage used/available, for instance).

I am attaching the configuration that we use in VG2. Most important changes are:

• added support for a few other useful stats (most importantly, storage used/free, download stats)
• different graph types where it makes sense
• example of setting warning/critical limits (storage free)
• an "introductory comment" which is hopefully helpful to make it easier to get started
• slightly different node naming scheme, and alpabetically sorted output of the nodes.

This might still need a bit of tweaking (especially the last bullet, node naming, already considers an idiosyncrasy of VG2, namely that VG2 has the convention of including the maintainer's mail address in the node name. This *should* only kick in if node names contain an "@", though). However, I think that it might serve as a slightly enhanced version of the current script which could be considered for inclusion in future releases.