mara on the tracking issue: "we need the swarm label for upstream otel at
least (the out of swarm one)."
Stamped in the per-hive `resource` processor rather than on a separate
upstream-only pipeline, which would double the pipeline count to withhold one
constant label from the local store. It is redundant there — one metrics store
per swarm, so every series in it already belongs to this swarm — but a constant
label multiplies no series, and it means what leaves and what stays have the
same shape.
Upstream is where it stops being redundant: that is the one hop where several
swarms can land in one store, and samples that cannot name their swarm collide
there exactly as hives collided here before per-hive receivers existed.
`unknown` when unnamed rather than an absent label, copying the agent path so
a query never has to handle both "the label is missing" and "the label says
unknown".
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