Set a RunTree as the active tracing parent within this block.
Unlike tracing_context, this only sets _PARENT_RUN_TREE and nothing
else, making it safe to use in isolated threads where you want precise
control over which run acts as the parent without inheriting or overwriting
other context variables.
| Name | Type | Description |
|---|---|---|
run_tree* | run_trees.RunTree | The RunTree to use as the active parent. |
