tracing::trace_state: add begin() overload for seastar::value_of given as a "request" parameter.
tracing::trace_state: add begin() overload for seastar::value_of given as a "request" parameter.

Signed-off-by: Vlad Zolotarov <976c0f8d0852ed6439ad273880b400a75a7c41d0@cloudius-systems.com>

raphaelsc/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla,duarten/scylla,avikivity/scylla,kjniemi/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,raphaelsc/scylla,raphaelsc/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,kjniemi/scylla