prometheus CPU should start in 0
prometheus CPU should start in 0

like the collectd metrics, prometheus CPU counting should start with
zero.

See scylladb/scylla#1727

Signed-off-by: Amnon Heiman <ed5fc74c4692051ad0b19749fd7cea6ec2d74b9e@scylladb.com>
Message-Id: <1475676222-26939-3-git-send-email-ed5fc74c4692051ad0b19749fd7cea6ec2d74b9e@scylladb.com>

scylladb/seastar,syuu1228/seastar,scylladb/seastar,cloudius-systems/seastar,cloudius-systems/seastar,syuu1228/seastar,dreamsxin/seastar,cloudius-systems/seastar,avikivity/seastar,dreamsxin/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,syuu1228/seastar,dreamsxin/seastar