More system metrics

[obmarg]

Once #1 has been merged, we'll have the initial reporting of system & vm metrics in. At some point it'd be good to add some more system metrics. Things like:

• Disk space free
• Number of inodes avaliable
• Number of file handles used/avaliable?

Some ideas from a TODO in my code:

    # /proc/loadavg -> system.load.1, system.load.5, system.load.15
    # /proc/sys/fs/file-nri -> system.fs.handles
    # /proc/stat -> system.cpu.x (or use mpstat if it's around?)
    # /proc/sys/fs/inode-nr -> system.inodes.total/used
    # disk space used/free?
    # /proc/meminfo -> system.mem (or something)