dashboard/app: add manager param to /cover?jsonl=1 output #16695

Job	Run time
aux	4m 22s
dashboard	1m 25s
build	3m 20s
arch (presubmit_arch_linux)	10m 35s
arch (presubmit_arch_freebsd)	6m 27s
race	7m 22s
gvisor	3m 57s
arch (presubmit_arch_openbsd)	3m 51s
arch (presubmit_arch_windows)	3m 46s
old	6m 56s
arch (presubmit_arch_executor)	3m 30s
arch (presubmit_arch_netbsd)	3m 55s
arch (presubmit_arch_darwin)	3m 53s
race_dashboard	1m 38s
	1h 4m 57s

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