Port cpu_monitor to ROS2 #480

Job	Run time
diagnostic_aggregator on humble	3m 23s
diagnostic_aggregator on iron	3m 38s
diagnostic_aggregator on rolling	3m 49s
diagnostic_common_diagnostics on humble	3m 22s
diagnostic_common_diagnostics on iron	3m 40s
diagnostic_common_diagnostics on rolling	3m 36s
diagnostic_updater on humble	3m 41s
diagnostic_updater on iron	3m 45s
diagnostic_updater on rolling	3m 25s
self_test on humble	3m 11s
self_test on iron	3m 57s
self_test on rolling	3m 27s
	42m 54s

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