feat(sdk-platform-java): Add CompositeTracer and CompositeTracerFactory. #18524

Job	Run time
generation-config-check	1m 43s
units (8)	42m 45s
enforcer	15m 13s
lint	3m 19s
units (25)	33m 2s
units (21)	33m 19s
units (17)	33m 20s
changes	5s
units (11)	43m 46s
windows	2m 15s
gapic-libraries-bom	21m 25s
split-units (sdk-platform-java, 8)	4m 21s
split-units (sdk-platform-java, 21)	4m 48s
split-units (sdk-platform-java, 11)	4m 22s
split-units (sdk-platform-java, 17)	4m 14s
split-units (sdk-platform-java, 25)	4m 12s
	4h 12m 9s

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