Godeps: update prometheus dependency

[yichengq]

prometheus updates its directory layout
(https://github.com/prometheus/client_golang#where-is-model-extraction-and-text)
and makes Godeps restore/save unable to work.

Remove all prometheus dependency manually and godep save again to fix
this problem.

/cc @timothysc

[timothysc]

I think the only way to verify would be to repeat the same commands:

go get github.com/coreos/etcd/etcdserver
godep save -r ./...

[yichengq]

@timothysc I have tested it and it could work. Moreover, we would recommend people to godep on our stable release, so i am backporting it to branch release-2.2.

[yichengq]

Merge it as-is due to low risk...

[timothysc]

updating my deps and it appears your 'godep -r ./...' on your local repository causes issues on anyone attempting to use, b/c it can't resolve.

Godeps/_workspace/src/github.com/coreos/etcd/storage/backend/backend.go:25:2: cannot find package "github.com/coreos/etcd/Godeps/_workspace/src/github.com/boltdb/bolt" in any of:
/usr/lib/golang/src/github.com/coreos/etcd/Godeps/_workspace/src/github.com/boltdb/bolt (from $GOROOT)
/home/tstclair/work/spaces/kubernetes/src/k8s.io/kubernetes/_output/local/go/src/github.com/coreos/etcd/Godeps/_workspace/src/github.com/boltdb/bolt (from $GOPATH)
/home/tstclair/work/spaces/kubernetes/src/k8s.io/kubernetes/Godeps/_workspace/src/github.com/coreos/etcd/Godeps/_workspace/src/github.com/boltdb/bolt

I literally get 100s of these.