refactor(ram_monitor): ros2 port #51

Job	Run time
diagnostic_aggregator on humble	3m 54s
diagnostic_aggregator on iron	4m 39s
diagnostic_aggregator on rolling	2m 19s
diagnostic_common_diagnostics on humble	4m 1s
diagnostic_common_diagnostics on iron	3m 6s
diagnostic_common_diagnostics on rolling	2m 31s
diagnostic_updater on humble	3m 47s
diagnostic_updater on iron	3m 28s
diagnostic_updater on rolling	2m 37s
self_test on humble	3m 31s
self_test on iron	3m 16s
self_test on rolling	3m 13s
	40m 22s

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