Context

It's not possible for a user to check the error code of an error passed by event.CommandFailedEvent without relying on the experimental API:

derr, ok := evt.Failure.(driver.Error)
if ok {
    fmt.Println(derr.Code)
}

An example use case are the metrics implementation for otelmongo which requires providing a database status code, particularly for failures.

Definition of done

The team has considered two solutions.

1. Add a Codes []string field to event.CommandFailedEvent
2. Support for this in the stable API, such as with the function mongo.ErrorCodes():

codes := mongo.ErrorCodes(err)
_ = codes

We should start with (1) and if that presents difficulties, attempt (2).

Pitfalls

We probably don't want to support a huge set of functions to parse information off arbitrary errors. Error codes feel unique, however, in that they can be acted upon.