Add metric for etcd binary version

[jpbetz]

The kubernetes community currently uses etcd-version-monitor to monitor which etcd version(s) are running across an etcd cluster. etcd-version-monitor polls the etcd version endpoint (e.g. http://localhost:2379/version) and translates the data to prometheus metric of the form:

prometheus.NewGaugeVec(
		prometheus.GaugeOpts{
			Namespace: namespace,
			Name:      "version_info",
			Help:      "Etcd server's binary version",
		},
		[]string{"binary_version"})

The metric is set with the binary_version label assigned to the current server version, e.g. 3.3.

Let's add this metric directly to etcd and eliminate the need for a version monitor process. This would be useful for any etcd operators interested in monitoring etcd version upgrades across an etcd cluster. With this metric they could easily set up a dashboard in their monitoring system (e.g. grafana) to visualize the various etcd versions. Each etcd server would emit a gauge value1 for it's current version and the monitoring system would aggregate the metrics from all etcd nodes in the cluster to produce a graph where the y-axis represents the count of etcd servers running each version.