test(spanner): skip flaky testMetricsSingleUseQuery #18759

Job	Run time
root-pom	1m 37s
owlbot-py	1m 4s
gitignore_correctness	1m 1s
generate-readme	1m 5s
gapic-bom	1m 37s
package_name_check	56s
group_id_check_for_maps_libraries	8m 48s
owlbot-yaml	1m 1s
	17m 9s

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