full diff: f4fb1b73fb...v0.9.4
version v0.9.0 is the minimum tagged version to work with go-metrics v0.0.1,
as it depends on `prometheus.Observer`:
vendor/github.com/docker/go-metrics/timer.go:39:4: undefined: prometheus.Observer
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>