Ported cpu_monitor to ROS2 #16

Job	Run time
diagnostic_aggregator on humble	4m 32s
diagnostic_aggregator on iron	3m 26s
diagnostic_aggregator on rolling	3m 56s
diagnostic_common_diagnostics on humble	3m 25s
diagnostic_common_diagnostics on iron	3m 8s
diagnostic_common_diagnostics on rolling	3m 13s
diagnostic_updater on humble	3m 41s
diagnostic_updater on iron	3m 6s
diagnostic_updater on rolling	3m 19s
self_test on humble	3m 45s
self_test on iron	4m 27s
self_test on rolling	4m 3s
	44m 1s

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