dashboard/app: show manager unique coverage #15252

Job	Run time
dashboard	1m 58s
aux	3m 48s
build	13m 46s
arch (presubmit_arch_linux)	9m 38s
gvisor	2m 58s
arch (presubmit_arch_freebsd)	5m 22s
old	5m 57s
arch (presubmit_arch_netbsd)	3m 21s
race	14m 57s
arch (presubmit_arch_openbsd)	3m 17s
arch (presubmit_arch_darwin)	3m 27s
arch (presubmit_arch_windows)	3m 22s
arch (presubmit_arch_executor)	3m 11s
race_dashboard	3m 28s
	1h 18m 30s

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