Ported cpu_monitor to ROS2 #15

Job	Run time
diagnostic_aggregator on humble	3m 51s
diagnostic_aggregator on iron	4m 10s
diagnostic_aggregator on rolling	4m 12s
diagnostic_common_diagnostics on humble	3m 30s
diagnostic_common_diagnostics on iron	3m 12s
diagnostic_common_diagnostics on rolling	3m 36s
diagnostic_updater on humble	3m 25s
diagnostic_updater on iron	3m 36s
diagnostic_updater on rolling	4m 0s
self_test on humble	4m 11s
self_test on iron	4m 7s
self_test on rolling	3m 44s
	45m 34s

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