dashboard: share Spanner clients #19171

Job	Run time
race_dashboard	2m 48s
build	4m 4s
race	7m 23s
old	7m 34s
aux	4m 3s
dashboard	2m 53s
gvisor	4m 7s
arch (presubmit_arch_openbsd)	3m 45s
arch (presubmit_arch_darwin)	3m 29s
arch (presubmit_arch_windows)	3m 34s
arch (presubmit_arch_linux)	10m 52s
arch (presubmit_arch_netbsd)	4m 3s
arch (presubmit_arch_freebsd)	6m 25s
arch (presubmit_arch_executor)	3m 27s
	1h 8m 27s

Provide feedback