Metric to count multi-document transaction write conflicts

We have a severStatus metric operation.writeConflicts that counts write conflicts that are retried internally in mongod, but that does not count write conflicts within multi-document transactions that are returned to the client and must be retried at the application level. Since retries are expensive it would be useful to have a such a counter to help diagnose performance issues that can result, for example maybe transactions.writeConflicts.