The journald receiver leaves the OTLP body empty and carries the entry as a
map of journal fields, so VictoriaLogs had no message to index and wrote a
placeholder into _msg on every record. Ingest returned 200, every field was
present, and a plain search for a line sitting in the store found nothing.
_msg_field names the field that holds the text. _stream_fields is the
difference between one stream for the whole host and one per unit per
machine; both are set by journald itself and both are low-cardinality.
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