api::storage_service: add set_trace_probability/get_trace_probability
api::storage_service: add set_trace_probability/get_trace_probability

Trace probability defines a probability for the next CQL command
to be traced.

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

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