feat: MetricsTracer implementation #4919

Job	Run time
build(21) except self-service clients	6m 18s
build (11)	5m 8s
build(8) for gapic-generator-java	3m 25s
build (17)	4m 55s
build(8) except for gapic-generator-java	3m 25s
compatibility	1m 23s
lint	28s
showcase (11)	4m 48s
showcase (17)	5m 5s
showcase-native	9m 21s
gapic-generator-java-bom	1m 29s
showcase-clirr	3m 11s
unmanaged_dependency_check	5m 8s
	54m 4s

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