≤ 10 entries; keys 1–64 chars [A-Za-z0-9_-] (no dots: a dotted name is MJ's source.field filter form,
which filters reject, so a dotted attribute would be unfilterable); values 1–256 chars (brokers refuse
empty values). The only fields filters see.
OptionalCorrelationUUID, globally unique (spec 03 §2.1). Stable across redeliveries and replays. Producer-supplied or generated at publish.
OptionalPartitionOrdering is publish order per key; there is no producer-supplied ordering number (spec 03 §1).
OptionalPayloadOptionalPayloadISO-8601 UTC, assigned by the publisher (MJ), not the producer.
The message as delivered to a handler. Identical on every transport.