Merge pull request #2500 from stripe/cory-fix-mesos-key-errors

[mesos_master] Fix key error when a metric is missing.

Author: Remi Hakim

checks.d/mesos_master.py

@@ -229,7 +229,8 @@ def check(self, instance):
                                self.CLUSTER_FRAMEWORK_METRICS, self.STATS_METRICS]
                 for m in metrics:
                     for key_name, (metric_name, metric_func) in m.iteritems():
-                        metric_func(self, metric_name, stats_metrics[key_name], tags=tags)
+                        if key_name in stats_metrics:
+                            metric_func(self, metric_name, stats_metrics[key_name], tags=tags)
 
 
         self.service_check_needed = True