Follow variable naming conventions of ROS2 style guide

[mm318]

For addressing the remaining issues in and closing out #50.

[codecov]

Codecov Report

Merging #56 into master will not change coverage.
The diff coverage is 28.22%.

@@          Coverage Diff           @@
##           master     #56   +/-   ##
======================================
  Coverage    41.7%   41.7%           
======================================
  Files          28      28           
  Lines         880     880           
  Branches      506     506           
======================================
  Hits          367     367           
  Misses         52      52           
  Partials      461     461

Flag	Coverage Δ
#unittests	41.7% <28.22%> (ø)	⬆️

Impacted Files	Coverage Δ
...llector/src/system_metrics_collector/collector.hpp	100% <ø> (ø)	⬆️
...llector/src/system_metrics_collector/collector.cpp	62.06% <0%> (ø)	⬆️
...em_metrics_collector/metrics_message_publisher.cpp	65.21% <0%> (ø)	⬆️
...r/src/moving_average_statistics/moving_average.hpp	100% <100%> (ø)	⬆️
...r/test/system_metrics_collector/test_collector.cpp	43.24% <20%> (ø)	⬆️
...etrics_collector/test_linux_memory_measurement.cpp	30.7% <23.4%> (ø)	⬆️
...m_metrics_collector/test_linux_cpu_measurement.cpp	28.8% <24.24%> (ø)	⬆️
...tor/test_linux_process_memory_measurement_node.cpp	41.93% <28.57%> (ø)	⬆️
...trics_collector/test_periodic_measurement_node.cpp	35.71% <31.57%> (ø)	⬆️
...rage_statistics/test_moving_average_statistics.cpp	59.22% <37.93%> (ø)	⬆️
... and 1 more

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