Prefix block attached to every log line. Rendered as
[<session_id> <request_id_or_-> <cluster_id_or_-> <backend_id_or_->]
so operators can grep a full TCP/TLS session (session_id) or drill
into one HTTP exchange (request_id).
Intermediate representation of an access log agnostic of the final format.
Every field is a reference to avoid capturing ownership (as a logger should).