dashboard: share Spanner clients #19172

Job	Run time
race	9m 15s
dashboard	2m 14s
old	6m 59s
aux	4m 7s
gvisor	4m 8s
race_dashboard	3m 32s
arch (presubmit_arch_netbsd)	3m 55s
build	3m 43s
arch (presubmit_arch_windows)	3m 42s
arch (presubmit_arch_openbsd)	3m 52s
arch (presubmit_arch_darwin)	3m 48s
arch (presubmit_arch_freebsd)	6m 27s
arch (presubmit_arch_executor)	3m 15s
arch (presubmit_arch_linux)	11m 4s
	1h 10m 1s

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