mara: "the subagent should already stamp the agent on the metric. so i want
to see count of distinct subagent names per agent name over time" — it
does. New timeseries panel-46 on the subagents tab: one line per parent
agent, counting distinct subagent label values with a turn in the trailing
$__rate_interval at each point. Rolling, not cumulative — can fall as well
as rise, same shape as any other rate()-derived panel here.
Placed at y:15, below where #4491's panel-45 (stat tile, y:11-15) sits once
merged, and numbered panel-46 rather than reusing panel-45 — argus caught
that this PR and #4491 both independently added a panel-45 in the same grid
slot from a shared unmerged base, which would have collided on whichever
merged second.
Narrower than a true concurrency gauge (subagents open at this exact
instant) — that needs hive-subagent-mcp to publish its own metric, which
nothing does today; noted as a separate, harder ask on #4488.
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