Per ratika.gandhi@mongodb.com, we'd like a way to detect if there are any cross-collection transactions in a cluster (for all kinds of commit, ie two phase, single shard, single write shard, and read only) and to see which collections are involved in any one particular transaction.

We don't have such a way today, but we already track the namespaces involved in a two phase commit (the _affectedNamespaces map in TransactionCoordinator), so we can possibly extend that to cover the other cases and expose the metrics somewhere we can collect.