Two questions on Ambari metrics:
you can leverage the rest api https://cwiki.apache.org/confluence/display/AMBARI/Ambari+Metrics+API+specification
and ambari metrics collector api https://cwiki.apache.org/confluence/display/AMBARI/Metrics+Collector+API+Specificatio
You can view metrics for an external system if you can add it as a "custom" service in Ambari and expose metrics.
This blog will be very helpful in achieving that.
Could you please explain what is an 'external system' and what would be running there? Yes metrics for all the installed HDP tools [master component] are available in Ambari.