[vsphere] skip unknown metrics

vSphere metrics are hardcoded and stored at in
`checks.libs.vmware.all_metrics`.

New versions of vSphere bring new metrics: thus, we should not consider
this list exhaustive, neither fail (raise a `KeyError` exceptions) on an
"unknown" metric.
```
2016-06-03 20:14:04 UTC | CRITICAL | dd.collector |
checks.vsphere(vsphere.py:932) | A worker thread crashed:
Traceback (most recent call last):
  File "/opt/datadog-agent/agent/checks.d/vsphere.py", line 299, in
wrapper
    method(*args, **kwargs)
  File "/opt/datadog-agent/agent/checks.d/vsphere.py", line 866, in
_collect_metrics_atomic
    if
ALL_METRICS[self.metrics_metadata[i_key][result.id.counterId]['name']]['s_type']
== 'rate':
KeyError: 'vflashModule.numActiveVMDKs'
```
Instead, skip these metrics.

Author: yannmh

checks.d/vsphere.py

@@ -844,12 +844,19 @@ def _collect_metrics_atomic(self, instance, mor):
                 value = self._transform_value(instance, result.id.counterId, result.value[0])
 
                 # Metric types are absolute, delta, and rate
-                if ALL_METRICS[self.metrics_metadata[i_key][result.id.counterId]['name']]['s_type'] == 'rate':
+                metric_name = self.metrics_metadata[i_key][result.id.counterId]['name']
+
+                if metric_name not in ALL_METRICS:
+                    self.log.debug(u"Skipping unknown `%s` metric.", metric_name)
+                    continue
+
+                if ALL_METRICS[metric_name]['s_type'] == 'rate':
                     record_metric = self.rate
                 else:
                     record_metric = self.gauge
+
                 record_metric(
-                    "vsphere.%s" % self.metrics_metadata[i_key][result.id.counterId]['name'],
+                    "vsphere.%s" % metric_name,
                     value,
                     hostname=mor['hostname'],
                     tags=['instance:%s' % instance_name]