Ported cpu_monitor to ROS2 #12

Job	Run time
diagnostic_aggregator on humble	4m 29s
diagnostic_aggregator on iron	4m 26s
diagnostic_aggregator on rolling	4m 22s
diagnostic_common_diagnostics on humble	4m 3s
diagnostic_common_diagnostics on iron	4m 7s
diagnostic_common_diagnostics on rolling	4m 24s
diagnostic_updater on humble	4m 10s
diagnostic_updater on iron	4m 2s
diagnostic_updater on rolling	3m 31s
self_test on humble	3m 41s
self_test on iron	4m 22s
self_test on rolling	3m 55s
	49m 32s

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