Loki metrics queries group only by indexed stream labels, so the
journal-as-JSON stream made every per-container view structurally
empty (measured 6.5k lines/5min flowing while the board read zero).
A loki.process relabel stage now extracts CONTAINER_NAME into a
'container' label. alloy 1.21's json stage has no on_error attribute
(validated against the pinned image) — format_as_json guarantees the
line shape, so no error branch exists.