dashboard/app: allow to fetch coverage from multiple managers #16610

Job	Run time
race	7m 13s
aux	4m 12s
race_dashboard	3m 48s
build	3m 58s
old	7m 44s
gvisor	4m 0s
arch (presubmit_arch_netbsd)	3m 50s
arch (presubmit_arch_darwin)	3m 49s
arch (presubmit_arch_openbsd)	3m 55s
arch (presubmit_arch_linux)	10m 34s
arch (presubmit_arch_freebsd)	6m 16s
arch (presubmit_arch_executor)	3m 28s
arch (presubmit_arch_windows)	3m 40s
dashboard	2m 50s
	1h 9m 17s

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