fix(gax): reduce visibility of ApiTracerContext methods #18993

Job	Run time
lint	3m 45s
windows	2m 45s
generation-config-check	1m 41s
enforcer	18m 22s
bulk-filter	1m 1s
gapic-libraries-bom	17m 50s
changes	3s
split-units (sdk-platform-java, 8)	2m 47s
split-units (sdk-platform-java, 25)	6m 3s
split-units (sdk-platform-java, 21)	5m 51s
split-units (sdk-platform-java, 11)	6m 21s
split-units (sdk-platform-java, 17)	6m 5s
units (8)	33m 38s
units (11)	34m 15s
units (17)	32m 24s
units (21)	31m 41s
units (25)	25m 39s
	3h 50m 11s

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