feat(sdk-platform-java): Add CompositeTracer and CompositeTracerFactory. #18529

Job	Run time
windows	2m 15s
changes	3s
enforcer	24m 32s
units (8)	42m 33s
lint	3m 5s
gapic-libraries-bom	21m 7s
units (11)	42m 51s
units (25)	34m 36s
generation-config-check	1m 39s
units (21)	31m 30s
units (17)	34m 29s
split-units (sdk-platform-java, 8)	3m 53s
split-units (sdk-platform-java, 25)	5m 12s
split-units (sdk-platform-java, 17)	5m 13s
split-units (sdk-platform-java, 21)	4m 1s
split-units (sdk-platform-java, 11)	4m 29s
	4h 21m 28s

Provide feedback