When using a Leaf txn, someone needs to collect its metadata and pass it through to the DistSQL receiver which merges it with the Root's metadata. A handful of processors (e.g. the TableReader) do this by collecting the leaf metadata when draining. However, this was only done for processors directly use the transaction.
But I believe any processor can use the transaction through its "render expressions" and such by invoking built-in functions. These functions can use the transaction through the EvalCtx. And so I think it might be possible for a txn to be used without anyone collecting its metadata.

Perhaps at the moment that's not actually happening because processors share their transaction objects (and also share it with the EvalCtx), but I'm moving towards less sharing. So the thing, if not broken already, is very fragile.

Separately, our collecting of txn metadata is very haphazard. It's possible for multiple processors to collect the same piece of metadata from a shared txn. We should figure out a more principled story.

Jira issue: CRDB-5476