fix(bigtable): fix session creation leaks #23579

Job	Run time
bulk-filter	1m 12s
gapic-libraries-bom	21m 28s
lint	3m 34s
units (8)	1m 5s
windows	1m 58s
units (17)	33s
enforcer	22m 36s
units (26)	34s
units (25)	30s
units (11)	32s
units (21)	30s
changes	29s
split-units (java-bigtable, 8)	6m 45s
split-clirr (java-bigtable)	3m 8s
split-dependencies (java-bigtable)	5m 7s
split-units (java-bigtable, 25)	8m 15s
split-units (java-bigtable, 26)	7m 50s
split-units (java-bigtable, 21)	7m 32s
split-units (java-bigtable, 17)	6m 38s
split-units (java-bigtable, 11)	6m 30s
conditional-required-check	4s
	1h 46m 50s

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