dashboard/app: show manager unique coverage #15250

Job	Run time
build	14m 10s
arch (presubmit_arch_linux)	9m 8s
dashboard	2m 6s
aux	3m 42s
arch (presubmit_arch_freebsd)	5m 31s
gvisor	3m 6s
arch (presubmit_arch_netbsd)	3m 26s
old	6m 2s
arch (presubmit_arch_openbsd)	3m 15s
race	14m 58s
race_dashboard	3m 18s
arch (presubmit_arch_darwin)	3m 11s
arch (presubmit_arch_windows)	3m 1s
arch (presubmit_arch_executor)	2m 53s
	1h 17m 47s

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