feat(sdk-platform-java): Add CompositeTracer and CompositeTracerFactory. #18525

Job	Run time
windows	2m 15s
enforcer	24m 5s
changes	3s
lint	3m 17s
units (8)	41m 5s
generation-config-check	1m 39s
gapic-libraries-bom	21m 18s
units (17)	33m 43s
units (11)	42m 10s
units (21)	32m 50s
units (25)	34m 43s
split-units (sdk-platform-java, 8)	3m 57s
split-units (sdk-platform-java, 17)	5m 40s
split-units (sdk-platform-java, 21)	4m 55s
split-units (sdk-platform-java, 11)	4m 19s
split-units (sdk-platform-java, 25)	4m 21s
	4h 20m 20s

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