Secondary tracing sessions used to compute the execution time
from the point of their `begin()`-ning, not the parent session's
`begin()`. As a result, replica reported a slow query if it
exceeded the entire threshold *on that replica* too.
This change augments `trace_info` with the TS of parent's session
starting point, to be used as a reference on replicas.
Fixes#9403Closes#10005
00a6fda7b9