fix(bigtable): avoid double close on pending vrpcs #19090

Job	Run time
gapic-bom	1m 55s
owlbot-yaml	1m 3s
generate-readme	1m 21s
gitignore_correctness	1m 19s
package_name_check	1m 5s
owlbot-py	1m 26s
group_id_check_for_maps_libraries	9m 0s
root-pom	1m 55s
	19m 4s

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