Private.TraceSourceTrace Eio events using OCaml's runtime events system.
Each thread/fiber/promise is identified by a unique ID.
Libraries and applications can use these functions to make the traces more useful.
log msg attaches text msg to the current fiber.
with_span op fn runs fn (), labelling the timespan during which it runs with op.
suspend_fiber op records that the current fiber is now suspended waiting for op.
These are normally only called by the scheduler.
create_fiber ~cc id records the creation of fiber id in context cc.
create_cc id ty records the creation of cancellation context id.
create_obj id ty records the creation of id.
get src records reading a promise, taking from a stream, taking a lock, etc.
try_get src records that the current fiber wants to get from src (which is not currently ready).
put dst records resolving a promise, adding to a stream, releasing a lock, etc.
suspend_domain records when the event loop is stopped waiting for events from the OS.
domain_spawn ~parent records that the current domain was spawned by fiber parent.
exit_cc () records that the current CC has finished.