[mdatagen] Add semconv reference for metrics in metadata schema #39380

Job	Run time
contrib-tests-prepare	5m 3s
contrib-tests-matrix (receiver-3)	9m 19s
contrib-tests-matrix (processor)	8m 15s
contrib-tests-matrix (internal)	11m 30s
contrib-tests-matrix (receiver-0)	9m 14s
contrib-tests-matrix (extension)	10m 47s
contrib-tests-matrix (receiver-1)	11m 48s
contrib-tests-matrix (exporter-1)	13m 20s
contrib-tests-matrix (other)	6m 5s
contrib-tests-matrix (connector)	6m 9s
contrib-tests-matrix (cmd-0)	2m 33s
contrib-tests-matrix (pkg)	7m 9s
contrib-tests-matrix (exporter-0)	4m 24s
contrib-tests-matrix (receiver-2)	10m 35s
contrib_tests	4s
	1h 56m 15s

Provide feedback