Trace()query_metadata describes why this trace matched the query, including
sem_filter_score when a semantic filter is used. Returned automatically, or
null when unavailable.
root_run is the trace's root run.
Which properties are populated is controlled by selects in the request.
trace_aggregates carries trace-wide aggregate metrics.
Omitted when no aggregate field was selected, or null (then later filled) on
the streaming wire while the aggregate values are still being computed.