Adding zpool/dataset IO statistics

[tuxick]

Hi,

I'm currently adding io statistics for zpools and datasets, but for datasets i'd introduce a dependency on:
iozstat. Before creating PR i 'm wondering if that'd be a big problem.
I think it's a nice tool to have around anyway :)

[tuxick]

A script to poll zfs iostats and write output for zabbix to read:

#!/bin/bash

OUTPUT=/tmp/zpooliostats

# number of pools 
POOLS=1
while true
do
        DATA=\`zpool iostat -Hp 60 2 | tail -n $POOLS\`
                # print output
                echo "$DATA" > $OUTPUT 
done

and then read that data:
vfs.file.contents[/tmp/zpooliostats]

after which in Zfs pool discovery have dependent items do:

zfs.bps.read[{#POOLNAME}]

with preprocessing regular expression:

{#POOLNAME}\s+([0-9]+)\s+([0-9x]+)\s+([0-9]+)\s+([0-9]+)\s+([0-9]+)\s+([0-9]+) and parameter \5

(and \6 for writes)