System resource usage collection

[carlossvg]

Hi @ahcorde @cottsay,

While comparing the ROS 2 performance test tools here there were some questions about different approaches used by each tool to collect the system resources.

It seems that buildfarm_perf_tests use system_metric_collector to collect the system data (https://github.com/ros2/buildfarm_perf_tests/pull/12), while performance_test uses getrusage (see analysis_result.cpp). I think it makes sense to re-use an existing tool such as system_metric_collector and join efforts to add new features there. First, I would like understand what are the pros and cons of each approach. Hopefully, this could be a step forward to reduce divergences between all the exiting tools.

cc @fadi-labib @SniderThanYou