Polling overall diagnostics state

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How do you find the overall diagnostics state from the diagnostics aggregator?

My diagnostics are running and viewable in RQT monitor, and now I'd like to make a simple rospy node that sets a status LED to red if there's an error, and green otherwise.

From the docs, I see there's a /diagnostics_toplevel_state topic, which I intuit should contain this info, but when I echo it, all I see is:

level: 2
name: toplevel_state
message: ''
hardware_id: ''
values: []

do all blank values mean everything is ok, or am I misinterpreting this? Is there a better way to poll the diagnostics aggregator?