Transport headers, populated on the 'messaging' arm from MessageMetadata.headers and on the 'queue' arm from IJob.headers. {} means the channel carried none; absent means there was no channel.

ABSENT on 'scheduler' deliberately, and that is a statement rather than an omission: a tick has no upstream request, so there is no header to carry and no trace to join. Adding the member "for symmetry" would erase the distinction the three states exist for — an operator could no longer tell "this work had no cause" from "the cause was lost".

ABSENT on 'websocket' for the different reason that a frame carries no per-frame headers; the upgrade request's headers are the connection's.