Trace identifier for the recorded trace. Treat the value as an opaque
handle — the current format is the on-device filename (e.g.
1713200000000.trace), but future versions may change it. Returns
an empty string if the profiler was unavailable.
Lens Studio retrieves the trace independently via the `script_trace`
file-pull request; a Lens does not need to interpret this value.
Stops the sampling profiler and writes the trace to device storage.