dashboard/app: allow to fetch coverage from multiple managers #16613

Job	Run time
build	3m 26s
aux	4m 19s
dashboard	2m 10s
race	8m 18s
arch (presubmit_arch_openbsd)	3m 53s
arch (presubmit_arch_darwin)	3m 53s
arch (presubmit_arch_freebsd)	6m 30s
arch (presubmit_arch_netbsd)	4m 1s
old	7m 9s
arch (presubmit_arch_linux)	10m 26s
gvisor	4m 1s
race_dashboard	3m 9s
arch (presubmit_arch_windows)	3m 38s
arch (presubmit_arch_executor)	3m 29s
	1h 8m 22s

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