[etcd] /stats/leader from a follower returns a 403

[yannmh]

The /stats/leader endpoint is only available on the leader etcd host.
On the followers, it returns a 403: it is illegitimately logged as an exception in the agent and triggers a CRITICAL service check.

#0[ERROR]: 'Http status code 403 on url http://localhost:4002/v2/stats/leader

Full stacktrace:

2015-06-17 16:41:46 UTC | ERROR | dd.collector | checks.etcd(__init__.py:678) | Check 'etcd' instance #0 failed
Traceback (most recent call last):
  File "/opt/datadog-agent/agent/checks/__init__.py", line 661, in run
    self.check(copy.deepcopy(instance))
  File "/opt/datadog-agent/agent/checks.d/etcd.py", line 114, in check
    leader_response = self._get_leader_metrics(url, timeout)
  File "/opt/datadog-agent/agent/checks.d/etcd.py", line 143, in _get_leader_metrics
    return self._get_json(url + "/v2/stats/leader", timeout)
  File "/opt/datadog-agent/agent/checks.d/etcd.py", line 159, in _get_json
    raise Exception("Http status code {0} on url {1}".format(r.status_code, url))
Exception: Http status code 403 on url http://localhost:4002/v2/stats/leader