Mongodb metrics fix

When running against hosts using Mongo 2.4.6, the metric would not return values due to an exception being thrown on the line:
```ruby
serverMetrics['indexes.missRatio'] = "#{sprintf(%.5f", serverStatus['indexCounters']['btree']['missRatio'])}"
````
In at least 2.4.6, there is no key for 'btree' any longer. A statement now checks for this key.