simple http stats endpoint

[spacejam]

desirable stats:

1. raft index for each etcd peer
2. is the last health check passing?
3. counter of reseed events since scheduler started
4. counter of etcd tasks launched
5. current etcd term for cluster
6. aggregated etcd stats (reads, writes, latency)

[jdef]

thinking of using prometheus?

[spacejam]

TSDB is external to this, and nothing we do should create a dependency on a particular TSDB IMO.

This implementation creates an http endpoint that can be queried by arbitrary metric collectors and then they can do whatever they want with them at that point.

Said external systems can query the /members endpoint and hit the etcd instances themselves for raft statistics. We can make this more featureful later on if there is any desire and need. https://github.com/mesosphere/etcd-mesos/pull/24