hive-c0re waited on hive-priv's answer with no timeout, so a hung
hive-priv would stall the disk sampler indefinitely. The wait is now
hive-priv's own 10 min bound plus a minute (the bound is shared as
MEASURE_AGENT_DISK_TIMEOUT); a timeout is an error, so the agent shows
unknown.
An agent with none of state/, work/ or the rootfs has no measurable
size. hive-priv now answers with an error for it instead of a total of
0, which read as a real measurement.
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