fix(gax): reduce visibility of ApiTracerContext methods #19033

Job	Run time
enforcer	18m 24s
bulk-filter	1m 19s
changes	6s
windows	2m 30s
lint	3m 7s
generation-config-check	2m 0s
gapic-libraries-bom	16m 58s
split-units (sdk-platform-java, 21)	6m 47s
split-units (sdk-platform-java, 8)	3m 10s
split-units (sdk-platform-java, 25)	5m 53s
split-units (sdk-platform-java, 11)	6m 40s
split-units (sdk-platform-java, 17)	6m 39s
units (25)	34m 21s
units (8)	32m 41s
units (11)	32m 53s
units (21)	32m 2s
units (17)	31m 38s
	3h 57m 8s

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