[aggregate-metrics] Adds ability to grab metrics for individual checks

Updates `aggregate-metrics.rb` to allow for grabbing the aggregated output metrics for an individual check. 

* Adds the option `--check CHECK`
* Adds the helper method `AggregateMetrics#push_metrics(check)` to handle outputting the metrics
* Updates `AggregateMetrics#run` for the new option and helper method.