dashboard/app: allow to fetch coverage from multiple managers #16606

Job	Run time
build	3m 19s
aux	4m 17s
dashboard	2m 24s
arch (presubmit_arch_linux)	10m 17s
arch (presubmit_arch_windows)	3m 38s
arch (presubmit_arch_darwin)	3m 47s
arch (presubmit_arch_netbsd)	3m 53s
arch (presubmit_arch_openbsd)	3m 45s
gvisor	4m 3s
arch (presubmit_arch_executor)	3m 22s
arch (presubmit_arch_freebsd)	6m 15s
race_dashboard	3m 44s
race	9m 21s
old	6m 52s
	1h 8m 57s

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